Since unRarX hasn't been updated in quite a while it still includes some fairly outdated versions of unrar and par2. Here's how you can manually update the binaries yourself:
RAR
Download the newest version of RAR (Download RAR for Mac - Command line RAR archive utility (Beta.) MacUpdate.com), extract it, and look for a binary file simply called "unrar". Now locate UnRarX.app in the finder, right-click it, and choose "Show Package Contents" from the contextual menu. From here navigate to the folder /Contents/Resources/, and copy your new version of unrar here, thereby replacing the old one.
Par2
To get a more recent version of par2 you may need to download an application which already contains a compiled binary. I recommend going with MacPAR deLuxe (Download MacPAR deLuxe for Mac - Manage and decode .par/.par2 files from newsgroups. MacUpdate.com). Download it, mount the DMG, and open the package contents as described above. Again, navigate to /Contents/Resources/ where you will find a binary file called par2SL. Copy it to the same directory in your UnRarX bundle, delete the old par2 binary and rename the new one to par2
If necessary, relaunch UnRarX. That it, you're set.