Why aren't all my songs showing up? [Rock Band (1 or 2)]
Straight out of the box your Rock Band disc should only have about a dozen songs available. By playing through different cities and venues in the World Tour mode (either solo or as part of a band) you'll be able to unlock the full track list. Once you've unlocked the songs in World Tour mode you'll be able to play them any time in the Quick Play modes.
In Rock Band 2, you can also opt to play challenges and unlock new songs on the disc that way.
Or, if you really want to be sneaky and unlock all of the songs for quick play there is an "unlock all" option in the modify game section (go to "extras -> modify game"). You will have to enter the code very quickly - rapid fire - for it to work:
Red, Yellow, Blue, Red (2), Blue (2), Red, Yellow, Blue
Please note that you cannot save your tour progress if you're playing with a game modifier turned on.