Well it takes the engine tuned right and the proper gear ratio. There are several threads here on both...that would be agood place to start....use the search.
Funny thing i bough a os18. started my 2.5 up today just to play around and give it a shot before i put my os in and it runs beautifully. You just need to get it tuned properly. You really need to regear for instant wheelies though. As mensioned before.
The Revo was designed to keep all four tires on the ground, this makes it a little harder to get it to wheelie. To get a stock 2.5 Revo to wheelie on demand would take a very low gear and a stiff rear suspension.
If you really want the on demand wheelies buy a stronger engine.