I need an NXT motor adapter
The adapter itself is generally called a "Molex" adapter. The one you have pictured looks similar to their Mini-Latch style.

I would recommend an adapter like this. The pin spacing should be the same even if it is not made specifically for that connector.

The red wire connects to pin 1 and the black wire connects to pin 2. The yellow wire is for speed feedback, but it won't do any good unless you modify the NXT firmware, so I would leave it detached.

Alternately, you could cut a NXT cable and solder it directly to the fan wires.

Also, you didn't say, but the fan is probably 12V and the NXT can only supply battery voltage, which is only at most 9V with fully charged batteries, so it will run slower than it does when connected to a 12V power supply.