As of December 2007, there were three parts to the map. You move from one part to the other by using the large orange arrows on the side. When you start a new session, you start in whichever part of the map you last used.
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Dinertown, far left: Winter Wonderland.
(You'll see the cruise ship in the lower left corner)
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Hometown, center: Zoo, Museum, Baseball, Boardwalk, Disco.
(You'll see the ferris wheel for the Boardwalk in the lower left corner.)
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The Wharf, far right: Coral Cove Waterpark, The Crypt, Hometown Harvest, My Diner.
(You'll see the My Diner sign on the left side, and an adobe mission building in the lower left.)
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Winter Wonderland seems to be the hardest to find. Go all the way left using the orange arrows until you're in Dinertown. Then look towards the upper left.
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To revisit any restaurant, click on its image on the map. The level menu at the bottom of the map will change to the levels for that restaurant. You can see your scores for each level and which ones you got Expert on. To replay a level, click on its level number, then click Play.
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One odd note: once you've made Expert on all 10 story mode levels for any given restaurant, you will unlock its Expert Level in the Expert Levels section. From then on, the highest score you get on the extra Expert Level will show up as your score for Level 1 of that restaurant on the map. This only matters to people who keep track of their scores over time, but it can be a bit confusing the first time you see it happen.

But I need to pay extra for purchasing those restaurants, right? (which I saw from the website)

The 5 diners in Hometown (Zoo, Museum, Baseball Stadium, Boardwalk, and Disco) and the My Diner in the Wharf come with the Gourmet Edition. The other optional restaurants cost $4.99 each. So far they've added about one new optional retaurant per month.
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Several people have used the example of Barbie. You buy the basic, and you can just play with that. Then it's up to you whether you want to spend extra to get more clothes, the car, the dreamhouse, etc. The Gourmet edition is the same way: it's up to you whether you buy the add-on restaurants.

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I thought it's a "full version" already.... that's why I got it... kinda not fair.... it sounds worse than those "extension sets" of other pc games....
If I know it has so many "add-on".. I may not purchase it...

The $19.95 game is definitely "a full version"--it has the same number of restaurants as previous DD games, plus if you get the Gourmet Edition (same price, but you have to buy it from playfirst.com) you get the multiplayer version and My Diner included.
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The add-ons are just that--extra things for people who want them. But you get the same amount of gameplay in DDHH without the add-ons as you did in DD, DD2, or DD3. So I think it's a good idea for people who want the extras, and it doesn't take anything away from people who just buy the main game.
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I thought The Crypt was a great add-on. Winter Wonderland didn't add much for me, though. So I may not buy the next add-on, at least not until I read some reviews and see what's in it. And I haven't bought any of the extra clothes, but I have a friend who's bought a lot of them. It just depends on what you get entertainment value out of.

Hi

I am unable to find how to access the multiplayer game thing. Also, there is no orange arrow on the main screen.

Can anyone help?

(Even though I am connected to the internet, it says you are playing in the offline mode)

Multiplayer is only available if you have the "Gourmet Edition" of DDHH, and you can only buy that version from playfirst.com . If you ought your DDHH on another site, it will be the standard edition, no multiplayer.

It will say Gourmet Edition on the main menu screen if that's what you have.

Oh, and I don't know if multiplayer is available on Mac or not.

I agree with you that the add ons are not vital. If I had not bought them I would still be busy replaying the real levels, because I never get tired of them. I did enjoy the add ons for a few weeks and I occasionally go and play them for a change, but they are definitely easier than the real game, and more of a novelty, so they don't attract our attention that much once the novelty wears off. Certainly, they are not necessary at all to enjoy the game. The same goes for the "expert levels" which we get for free - a cute challenge, but not a vital part of the real game.

Win ,Please!

Couple of notes...
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Yahoo now carries the Gourmet Edition also.
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The "time travel" restaurants are available all on one screen that has a space galaxy behind it. You get there by using an orange arrow to go UP from Dinertown, which is the section to the left of Hometown.
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Also, members who also have Wedding Dash 2 have reported that when they have it installed also, there's a wedding chapel somewhere in Dinertown that has a bell on the top of it, and when you click on it, it starts Wedding Dash. Pretty cool.