This is the cheese that I use and I find it at our local Wal-Mart store. Cut the cheese into cubes for easier melting and put into a microwave safe bowl. To the cheese add 1/2 cup of milk. (The package calls for more, but I find 1/2 cup works best). Microwave 45 seconds at a time and stir in between times until it's creamy.
Next I take some jarred salsa and run in through my blender on puree to make it smooth. I hate chunky salsa, plus it makes it taste more like the restaurant salsa.