This can occur when a portal isn't enabled for publishing with metaPost.  Enabling portals is on the honor system and we've configured the portal management to give customers a rudimentary way to re-sell the metaPost service on a portal to customers. 

To see this admin screen, simply log in with a host account and add the metaPost module to an admin page (any page will work).  Once the metaPost module is added to the page, select Administration from the actions menu.  In the screen that appears, click the link to manage portals.  This will allow you to quicly see which portals in your instance of DNN have metaPost enabled. 

If this doesn't solve your problem, let me know and thanks for using metaPost!

Don