Apparently this is a common problem. After upgrading Logic from 7 to 8, things get broken. During the upgrade things get moved around and because logic depends on a lot of garageband content, some of these files ended up missing. I looked for some way online to fix the problem. Most said to reinstall Logic or to use a program called pacifist to dig out the needed files from the Logic discs. I decided that if it is missing Garageband content, why don’t I just reinstall it. I uninstalled GarageBand recently because I don’t use it do reinstalling it was easy.
If you have missing EXS files in Logic Studio, try reinstalling GarageBand. That might fix it.