Not much info in your post.
Assuming you are using the mr16 without the mosfets, you do need different firmware. The mosfets not only increase the current capability, they also invert the logic...
Be aware that the maximum current capability of a mr16 without the mosfets is very low. I was looking at this and even made one that way before really understanding how little power the TI part can handle without some help...
By the way, even with the mosfets, you are sill doing constant current. The circuit is either full on or full off and a resistor is used to set the current. You get different brightnesses by varying the ratio between the "on" time and "off" time at a speed faster than the eye can process. The eye then averages the total amount of light received...