i used the "Rectangular Patterns" button on the tools menu...

i also had some strange things happen when i tried importing png-format. but once i had loaded it into fireworks and saved it as png 32, it worked fine. i guess the png was in some old format that the editor didnt read properly, or something? (it was a strange black/white/negative).

NB: in my opinion, its more important that you get the tileengine working than fixing this