I think that could be a stupid question but I don't care:
When I want to open these .ftm or .nsf attachments, I get an error popup that says that I ought to send an error report to Microsoft. Did I forget to download something that I can't open these files or what is the problem?
There's no such thing as a nsf2ftm converter. NSF is the playback format, not the editing format. The only thing you can do with NSF files is listen to them.