Here's a step-by-step guide for those who have tried everything and are still having no luck.
1. Log in as an administrator. How do you know if you're an administrator? Go to System Preferences and open the User Accounts pane. You should see something that looks like
this (though possibly with more accounts). Note the keyword "Admin" underneath the name under "My Account."
2. Delete (or put off to the side somewhere) everything remotely associated with Planeshift, including the application, the DMG, the settings files, everything. If you do store this stuff somewhere instead of deleting it, do not touch it at all during this process.
3. Download
this. Safari may say "decompression failed." That's ok. Just put it on your Desktop (if it's not there already) and double-click it. It should decompress to a folder containing two files: 1. Updater-old.jar and 2. updaterconfig.xml (and yes, 10.4.x users, you can use Updater-old.jar without any problems).
4. Double-click Updater-old.jar to open the updater. Ignore all the options and just click "Start Update." Wait for it to download the game (it's gonna download EVERYTHING).
5. When it's done, close it and double-click psclient(.app). The game should run fine without a problem. If it doesn't, please send me a PM including your system specs: OS (i.e. 10.4.6), Model (i.e. MacBook black), Processor (i.e. Intel Core Duo 2: 2.2 GHz), and RAM (i.e. 1024 MB). If you encounter any errors during the update process, please send me your updater log as well.
I hope this helps everyone.

Thanks for bearing with us!