You can self rate your Xanga site on the SafeBrowse Preferences page:

http://www.xanga.com/private/editxangasafebrowse.aspx

Scroll down to the section marked "My Xanga Site" and select the appropriate rating from the pulldown.

You can also self rate individual posts.  For a new weblog entry, just select a rating from the pulldown near the bottom of the Editor screen before you submit the entry.  To edit an existing entry, click the "edit it" link beneath the entry, select a rating, and save your changes.  That should do it!