Ah. I'm not at all familiar with iMacs, so I'm afraid someone else will have to help out here. I don't know what DSS is, and I don't know what file format the DM-20 uses for recording. In terms of the file size, perhaps it would be a better question if you described in some additional detail the procedure you are following for transferring the recorded lecture to your Mac, i.e., is the file you're trying to import located *directly* on the DM-20, or have you transferred a file from it to your iMac? Or is the DM-20 connected as a removable device to your iMac via USB or FireWire? We're definitely getting closer to solving the problem; offhand, it still strikes me as if the recording was made in a file format that Audacity does not or cannot support directly. You'll have to specifically identify the file format the DM-20 uses, determine whether Audacity supports that format, and then investigate whether there is a conversion utility which can reformat the file into something Audacity can import.
The most important question - that of the file format of the recorded lecture, remains unanswered - and is probably the key to understanding this entire issue. What I'm asking is whether the recording's format is MP3, WAV, MP4, AIFF, AAC, FLAC, WMA, M4A, etc., etc.