I'd cut that plastic piece off then wire the + with +, and - with -.
A wall adapter with an output of 12v and 2A (2000ma) is what I use to power two 160mm fans, just make sure the combined "ma" rating for your fans is lower than the output of your plug, and that the plug has an output of 12v hth.
I also use a Garmin GPS Charger to power two 120mm (intake) fans