When is this game coming to stores in the U.S or is it already here? If so, where can I find it because I would like to play it but my computer won't let me download it.

As of now, it's a PF exclusive, so no one else has it. After a few weeks, check out www.retro64.com or one of the other major portals and see if they've acquired it yet. It takes even longer usually for download games to go to retail.
If you're having download problems, try disabling your firewall and antivirus software before downloading from PF. It might let you download it. :D
Hope this helps!