Open the box correctly and take it out. See if the neck of the airfilter fits onto the seal on your motor. If not, then it won't work. Take it back and ask them for a air filter that will fit the .15 motor from Traxxas for your N. Stampede.
As long as you don't break, disassemble or manage to harm the purchased air filter they'll take it back or exchange it for one that fits perfectly.
Cheers,
Donzii.