So, I checked out Zex's website. They recommend injecting nitrous before the MAF with a dry shot.
Step 5: Install Nitrous Nozzle
Optimum nozzle placement is 6" to 18" from the throttle body. After you have determined where to mount the nitrous nozzle (Fig. F) in the intake, make sure this location won't interfere with other components. If your engine uses a mass air flow sensor, mount the nozzle after the sensor, but before the throttle body.