Download the latest version, v0.5.16, from https://multibit.org/releases/multibit-0.5.16/multibit-0.5.16.dmg.

Once you've installed that, see if it fixes the problem. If not, the next step would be to try backing up and deleting (or just moving/renaming) the settings and other files, but an update may fix the problem, and if it does, it's easier than going through the process of moving the files out of the way.