Working in the graphic design field, you learn a lot of handy little tricks for making artistic life behind a computer a little easier. I'd like to share these with you.
This particular problem was a real pain in the neck, and after searching things like "OSX Won't Install Fonts" and "OSX Cache Out Fonts" on Google, I couldn't find anybody who had the same problem.
Apparently, Mac OSX Snow Leopard hates it when you mess with its font cache. After using the cache-cleaning software "Cache Out" as per a suggestion from somebody at work, I found that I couldn't install new fonts on my work computer. After reinstalling OSX at work, I found that I had an identical problem on my home Mac. Here are the programs involved in both cases:
Using Font Book
Adobe CS4 Suite
Using Font Explorer X
Adobe CS4 Suite
I had been convinced that this was a Font Explorer issue or a corrupted font until I found out that my home machine was misbehaving as well.
At work, Font Explorer X showed all non-system, non-Adobe fonts as "Unavailable." None of my Adobe software could find anything, save for Adobe's fonts and the System fonts.
At home, Font Book showed all of my 300 some-odd fonts as being installed, but none of the applications on my computer (CS4 software, OpenOffice, etc) could find the fonts.
Make a new User Account on your Mac. If everything's hunky dory there and you can install fonts, you can be pretty sure that your preference files are borked.
Condensed Tutorial: Make a new user account to allow OSX to write you some clean preferences, copy and paste these new preferences over your old preferences.
Step One: Back up your preference file, found at Users > (Your User Account) > Library > Preferences. Copy and paste all of these files to somewhere safe, like your desktop or a thumb drive.
Step Two: Get a thumb drive, external drive, or some other kind of media ready. Make a new user account. Go to your System Preferences, click Accounts, and if necessary, click the little padlock in the lower left of this window to make changes. Click the plus-sign below the listing on the left ("My Account," "Other Accounts,"...) and go through the steps to make a new account.
Step Three: Log into your new account. Go to Users > (New Account User) > Library > Preferences, and copy these files to your external media.
Step Four: Log out of your new account. Log into your regular account, and copy all of your new preference files from your external media. Paste them into your Users > (Your User Account) > Library > Preferences folder.
Step Five: Bask in the glory of your fonts working again.
Note: Keep all of your old preferences handy, in case there was something in there that you needed.