Mango, you should just be able to reinstall the app, then use the "Set up Sync" button on launch to point OmniFocus at your existing sync database.
Just re-enter your MobileMe user name (not the full email address, just the stuff to the left of the "@me.com") and you're likely going to be good to go. By default OmniFocus knows to look in the Documents folder on your iDisk for the sync database. If you see anything unexpected happen,
send us email or give us a call (800.315.OMNI) and we can help.