Your question is incomplete.
Try trashing the prefs which is always required after an upgrade of OSX. Start by quitting Lightroom 3.
Launch Finder and click the Go menu whilst holding down the Option (alt) key. Then go to Library >> Preferences and clear everything relating to LR3 which usually means deleting the following files:
Then re-launch Lightroom.
This assumes you have upgraded your Mac, if not, let us know