Sparkfun MOSFET Power Control Kit (https://www.sparkfun.com/products/10256), and clearly the constant-current mode did not like it and the output current did not stay stable.
You do realize that that particular kit is not a constant current driver, right? However, if you add a small npn + a resistor to it, you can easily make it into a constant current drive.
Alternatively, you can find a switching mode power supply and pwm its on/shutdown pin to pwm dim the leds.