Near the point were the seat back mounts to the seat frame, there is a little lever you have to life to get back the tab welded to the seat track.
That may be for new seat. So, If thats not there, then you have to move the release lever about a 180 degrees. That lever, during normal operation, just move about 90 degrees. Enough to release the seat and slide it. But when you want to remove the seat, you need to move it enough to slide past the tab on the seat track.
Either way, there is a tab on the seat track you have to get past.