Reply to 4TRAK Custom Mapping Help

Hello! Sorry for the somewhat noobish question, but I'm having a hard time mapping the "active" button on my 4TRAK to Traktor 2.6 (it's this button here: http://i.imgur.com/4oTRgT3.jpg).

I can't find the corresponding button on the 4TRAK controller so I want to customize it to one of the assignable buttons. Thanks so much in advance!