Sorry for the trouble here! Unfortunately, OmniFocus does not yet have the ability to merge two different databases. Let's try the following:
If you go ahead and pick the Sync option in that dialog, that will overwrite the iMac's database with the one on the server. However, the iMac's database is backed up before this happens.
If you open your Backups folder in Documents/OmniFocus Backups (or Library/Application Support/OmniFocus/Backups if you're running the Mac App Store version), you should see a backup file called "OmniFocus prior to resynchronization". Double-click that file to open it in OmniFocus, go into Context mode and open the View Bar, then set the grouping and sorting filters to "Added". This will give you a list of your actions sorted by when they were added (in order of most recent to oldest). You can use the same filters on your main database to compare the two, then copy and paste any data that may be missing from your main database. Then, run a sync with your server, and you should be all set.
We'd like to have the ability to merge databases in the future. If you're still having trouble, don't hesitate to
email us or call 800.315.6664. Thanks!