Are you playing on the same console that you used to download the map pack? If not, are you signed in to Xbox Live?
A piece of downloadable content is licensed to the first console that downloads it and to the Gamertag that purchased it. If you took your hard drive to someone else's console to make the download, the item was licensed to their console, not yours. You'll have to sign in to Xbox Live in order to play it on your own console.
This can be fixed by migrating your licenses to your console through the Xbox website, but it is time consuming and you will need to connect your console to Xbox Live to obtain the license.
If you wait long enough, Cole and his partner will figure out the clue and point you in the correct direction. If you haven't visited that landmark yet, it will appear as a ? on your map, just like the special cars.
The Desert Eagle is pretty ubiquitous in warfare games set in the modern day. Even games that only have a few pistols tend to include it, so I don't see why Modern Warfare 3 would leave it out.
Personally, I'm not a fan. The recoil on those monsters hurts like mad. I'm a 1911 girl.
New Austin is the Red Dead equivalent of Texas. It includes all land between the two main rivers, from Thieve's Landing west to the edge of the map.
Thieve's Landing, MacFarlane's Ranch, Armadillo, Rathskellar Fork, and Bennedict Point are some of the notable areas in this region.