You can export the constraints into a UCF file which you can then use for the design. Just go to File -> Export Constraints and follow the directions.
Or if you're wondering what commands are taking place when moving things around graphically on the floorplan have a look at the Console tab at the bottom. This will show you the commands being used. These are mostly Tcl commands and you can't use them to affect a generic UCF file, but you can take note of them and use them in another PlanAhead project.