You can't post to metros.  It's just a way to connect the weblogs of members who are associated with the same geographic area..

Sometimes Xangans look at the metros page, and ask us about those messages that appear under Xanga usernames.  That's actually just the About Me info from your profile.

For more info on how to change that, please see:

How can I edit the message that appears under my name on the metros page?
http://help.xanga.com/editmetrosmessage.htm