How to wire up a logic-level MOSFET to a PWM Signal?
The basic circuit you need is this.

Your N Mosfet is switching the load (the LEDs) to ground. R1 limits the initial gate current (gate acts like a capacitor to ground) and R2 discharges the gate ensuring the mosfet is completely off if disconnected from the arduino. The 0V of the battery needs to be connected to the 0V (ground) of the arduino