Thank you for letting us know - and very pleased to hear that you do indeed have iMovie '11 on your iMac.
But, as AppleMan mentioned "a new Mac would never have 10.6.8 installed". That's known as Snow Leopard. There have been two major upgrades since Snow Leopard, being Lion (Mac OS X 10.7) and Mountain Lion (Mac OS X 10.8 - the latest update being 10.8.3).
I purchased my wife's iMac in October 2011 (that's 19 months ago). Whilst that same model was selling with Lion installed, the store did have one available in stock that hadn't been upgraded to Lion, still having Snow Leopard installed. My wife preferred to stay with Snow Leopard as she had a number of earlier apps (Power PC) that she wanted to continue using (they don't run under Lion and Mountain Lion).
I think you should still take this up with the Store to determine why you've been sold a new iMac with a quite old operating system (Snow Leopard), and why an extremely old (2006) version of iMovie was installed along with the latest version. It sounds like you may have been sold an ex demo model that has been reboxed. Possibly, the box was grabbed off the shelf inadvertently, but you need to get to the bottom of this.
On Snow Leopard, I think that iMovie '11 will run only up to version 9.0.4 (the latest is 9.0.8 which runs on Mac OS X 10.7.4 and later). Can you tell us what the version number of your copy of iMovie '11 is please? If you click on it in the Applications folder in Finder column view, you will see the version number listed in the next column. Otherwise, in iMovie click on iMovie then About iMovie.
Enjoy movie making with iMovie '11.
John