Contact SRAM - Truvativ directly ....they can help you
REsponse from SRAM:
As you've found out, The ISIS interface (left arm you purchased) is different than the GXP interface (of your crank spindle), even though they appear similar.
The crank arm you need is our part number 11.6115.287.000. Your favorite local or online SRAM dealer should be able to source this for you.
Thanks to y'all for the help.
Gyro