Adjust the "Ceiling" control on either the software's graphic user interface, or the front panel controls. This sets the peak level of the audio that will be output from the limiter. Getting the mix as close to 0db as possible is ideal. Setting the ceiling above 0db will cause the mix to clip and become distorted.
Use the "Gain" knob to increase the overall perception of the sound. Keep in mind that the more "Gain" you use, the more the sound will become compressed. Over compression will destroy the range of dynamics, leaving your track or mix sounding thin and hollow. It is best to boost the instrument's audio signal in the mixer before sending it to the limiter, instead of adding extra processing while limiting.
Change the "Lookahead" setting to higher increments if you want the limiter's processor to pre-scan the audio input, giving a estimation for the correct processing ahead of time. If you want the limiter to use less CPU resources, set the "Lookahead" to lower increments.
Adjust the "Release" knob to determine how short the effect's processing time will take to complete its cycle. For drum tracks, shorter releases create a tight effect. For instruments like pads, or whole mixes even, longer releases may fit better.
Go back between the affected and non-affected tracks by disabling the limiter. This will give you a better idea as to how the original sound is being affected by the limiter. All of these controls mentioned are standard in both software and hardware limiters. When used in modesty, they can give added perception of sound as well as keep it from from sending a clipped signal through the mixing bus.