You should not have to file the spools down. The little knobs on the side of the camera that align with the spools are spring loaded. Pull them out and it will release the end of the spool, or allow a new one to be placed inside.
I know this is old, but to anyone here in the future like me, Andrew Moxom is correct. You simply must twist and pull the on the top left side until you create enough space to drop the spool in. After I did that, my spool went in with no real issue.