Wow same question twice in the same day on two different forums.
Heres the answer I gave in the other forum...the poster wrote back that it worked for him:
I'm not sure if it tells you anywhere the max number of characters for a GUID, but I think you need to type the MAC with no colons or hyphens, and preface it with the number of zeros needed to reach the max chars.
e.g. 000000000000000004AC5D0590 or whatever, where the MAC is 00:04:AC:5D:05:90