Judging from the error you're getting, I'd say you're not doing anything wrong. The format of the download doesn't matter since it outputs in it's own file format.
My guess is that you have an early build 18TA and that you're still running on EPROM, which will require a new EPROM and the EPROM burner to make an edit.
Typical memory card output for FAPTIII regardless of RS232 or PCMCIA should output as either pmc.000 or pmc.lad
(Note: The opinions expressed in this post are my own and are not necessarily those of cnc-professional-forum.com and its management)