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I think that Excellon outputs drill coordinates in ascii for use by the fabricator's machinery. So there is no Gerber drill info to put into Viewmate, I think. I've generated drill gerbers by mistake before and they showed up no problem in viewmate, but when I corrected to excellon it doesn't import because its not supposed to.
Your excellon drill file should be a TXT file. If you are trying to import the .dri file, that's the wrong one.
In viewmate select a new layer and select file->import->drill & route. Select Options on the bottom of the open screen that comes up. You may have to fiddle with this. I changed the default for zero's to 'all digits present' and changed the units to 'metric' and the file imported correctly.