We rode the WAX after a 3 day stay at the Monte Carlo. It was a short 10 minute walk from the Monte Carlo to the bus stop, just be sure to cross over Las Vegas Blvd (from NYNY to MGM) and then cross Tropicana Ave (from MGM to Tropicana). The bus stop to the airport is on Tropicana Ave in front of the VIP / Locals parking lot.
Just keep in mind that other buses stop here, so wait for the one that says Westcliff Airport Express. There is no outdoor ticket machine at this stop so make sure you have exact change ($2 in coins).
The trip to terminal one is 10 minutes.