Inside of the package, you'll find a dtregister_falang.zip file. Install this in the Falang content elements area. This will add 5 content element files to use. This was just added to the package a few days ago, so if you don't have it, download again.
As far as migrating Joomfish data to Falang, I'm not sure. You need to ask the Falang team about that. Our 2.5 content elements are modified versions of the old Joomfish ones so it might be possible. You'll just need to confirm with them.