Simply place an order at the Byonics website: http://www.byonics.com/microtrak/mtaio.php
. You want the MicroTrak AIO version 2, the one with the rubber duck antenna. Do not get a mag-mount antenna.
When you place your order, give Byonics your ham radio call sign, and if you want, an SSID number 1 to 12 which would differentiate your beacon if you are using more than one with the same call sign (Ex: KJ6MOT-2 or KJ6MOT-11).
I also found that the rubber duck antenna did not transmit the signal very well. I opted for a high altitude dipole antenna from Byonics. This was great for our flight. We mounted it on the side of the payload, and everything worked out great. http://www.byonics.com/microtrak/mt_antenna.php
UCSD near space has some pretty good details on the MicroTrak as well as some other great resources. https://sites.google.com/site/ucsdnearspaceballoon/documentation/how-to-use-the-microtrak-aio-beacon