For most Nikon DSLR bodies flash exposure compensation (FEC) can be adjusted by pressing and holding the flash button and then turning the sub-command dial (located near the shutter button) until the desired FEC is selected.
If your Nikon does not have a sub-command dial, then press and hold the flash button and the exposure compensation button together.
While still holding the two buttons, turn the main command dial (located on the back of the body) until the desired FEC is selected.
The adjustments will be shown on the screen
For Graslon Flash Diffusers we recommend setting the FEC to +1 2/3 to +2.
Most Nikon bodies limit FEC to +1. In this situation we recommend increasing the exposure compensation +1 as well. The can be changed by holding the exposure compensation button and turning the main dial. Alternatively you can set the flash compensation on the flash instead of on the body.
If this does not work for your particular camera, please consult the manufacturers instruction manual.
Be sure to set the FEC back to zero when not using a diffuser.