have you tried using the STOCK SMS app to send and receive MMS messages? this will determine if its an issue with the phone or with that 3rd party app.
if you tried the stock app and you cant receive mms there, then contact your local cellular provider to make sure your account is setup properly.
if you can receive mms messages from the stock app, then the issue is with the 3rd party app and a ping to the dev is in order.
since you are rooted (even though you run stock os, could still be something tangled there), lets move the discussion to the rooted users section.