If you are a free member of Xanga, unfortunately, you cannot blog on Xanga anymore, unless you pay a subscription fee. https://xanga.crowdhoster.com/relaunch-xanga
If you wish to pay for a subscription, your account will be upgraded so that you can upload your archive.

That being said, if you do not wish to remain on Xanga, there are a variety of other bloghost providers, that offer service for free. Since you have downloaded your archive, you are free to choose to go wherever you want. The easiest of these, would be Wordpress.com since your archive is already in the proper format. You'll want to extract your files out of the ZIP folder, and this will give you XML files of your blog. If you set up a new blog on Wordpress.com -- simply follow the import instructions for each XML file: http://en.support.wordpress.com/import/ Make sure to select Wordpress as your import option. Other bloghosts may have their own unique importing methods, and may require conversion tools or other steps to move.