I just called in to activate this plan (I'm already on a prepaid TMO plan). I couldn't get it on the phone, but the CSR told me that I could order the activation kit from walmart.com, call in, and activate it using the code from the kit. The code can be applied to either a new line or an existing line. The kit sells for ~$30 but includes $30 of service.

Here's the kit: http://www.walmart.com/ip/T-Mobile-SIM-Kit/24099996