the 545 is for single sheet film. this has nothing to do with pack film. the fuji packs have exactly the size of the old polaroid packs. if the pack slips in, it fits. the only difference is that fuji has 10 film in one pack, the old polaroid had only 8. which makes the pack well packed. so for certain backs it's difficult to tear the black shield and sometimes the first film. after this it's like the old polaroid packs.
are you shure that you put the pack and the white tabs the way it should be? there are plenty of videos on the web which show you how to do. I've seen people trying to get the black slip between the rollers (not that I presume that you did). the white tabs are so important. if the rollers are clean and one of it moves easily, the yellow tab should appear after you have pulled the white tab. and then by pulling the yellow tab straight out of the back it should work.