Q:

When do you place the Mezuzah? 

Possible Duplicate:
Mezzuzahs- putting them up, and saying brachot… 

When do you place the Mezuzah?  30 days after beginning rental or moving in? 

A:

See the section from Oholei Yeshurun that I quoted in this answer.  The language used there for when to start the clock is "upon entering."  I expect that that refers to moving in, not to any off-site formal transaction.