To add a new entry, just sign into your Xanga account. You should end up here:

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

There should be a link or button near the top of your page that says either "New Weblog Entry" or "Add Weblog"

When you click on it, an editing interface will appear. Just write your new entry, and then click "Save Changes" to publish it to your Xanga site.