OK - if you're like me you've had no help from these boards and from the myriad of "Hi, I am Straight Talk Ricky, I send you a PM" that goes nowhere. If you have an ATT compatible Nokia Lumia Windows Phone and you need APN settings, these are the ones I'm using (below). I couldn't get MMS and Straight Talk CSRs couldn't figure it out and kept telling me it was my phone, which is right out of the "customer no service" playbook.
In your APN settings, make sure you set the Authentication Type to: PAP
This is what I needed to activate in order to get MMS, at no time did a CSR recommend modifying this setting.