User KE7GKP is correct. According to the datasheet you linked to, the Dimming input pin, Pin#2 "Can be connected to a logic level PWM signal". (Typical logic levels are 3.3V or 5V)On page 3 it says Vdim threshold ranges from 1.85 to 2.25 typically 2. Therefore when the voltage is below 1.85v it is off and when it is above 2.25v it is on (or vise-versa). This confirms you can use either a 3.3V logic or 5V logic. So the arduino output will be fine. No voltage divider is required.
@neurostar: "In theory, driving power LEDs in parallel is a bad idea." The design being worked from the first page of the thread had current limit resistors in each string, thus they are not actually in parallel.