We discussed it here …
Getting unique ID of a user's machine
… and here …
http://sketchucation.com/forums/viewtopic.php?p=557992#p557992
On windows, you might just run …
%x[ipconfig /all > somefile.txt]
… and then parse the output for the line with “Physical Address” following after the line which begins “Ethernet adaptor”
Note: I show redirection to file because every few SU versions the execute string (global backquote method,) gets broken by SketchUp’s Ruby Console implementation (on Windows editions at least.)