The eDrawings client EPDM uses (on a 64bit machine) should be in C:\Program Files (x86)\Common Files\eDrawings2012
in there is a program called eDrawingOfficeAutomator.exe that you can run from command line with the /regserver switch to set it up as the correct viewer.
More info on this can be found in the knowledgebase under solution S-013202
Also, be prepared to run it multiple times, and make sure you're at an administrative command prompt. I usually have the most luck running it right after a restart before doing anything else, don't open the vault or anything, but that's no guarantee.