To uninstall LW, just drag & drop the program into the trash. If you wish to downgrade to an older version, then you may need to remove a install package file. You get the "null" message in the Apple installer when a OS X thinks a more recent version is installed. Double-click the harddrive icon, open the Library, open the Receipts folder, then find the file: Install LimeWire.pkg, delete it and try again. Receipts folder sample image

It is probably worth deleting your LW preferences folder whilst LW is closed. Found here:
/Users/your_username/Library/Preferences/LimeWire

Sorry for my extremely late response.