To upload a KMZ file, click the "Upload" button in the upper right-
hand corner of the home page (just under the sign-in). That will take
you to a form that lets you enter the KMZ filename (as well as other
information).
You might want to read over the help center article on uploading KMZs
as well, which describes the requirements for a valid KMZ file:
http://sketchup.google.com/support/bin/answer.py?answer=76529&hl=en
Cheers,
Lori