Open the song that you want to alter.
Click Window, then Mix.
Click the Insert Selector on the track (the left side top) and choose Compressor/Limiter from the menu.
Start playing the track and click Librarian, then choose one of the preset settings to see how it affects the track. When you find one you like, click OK.
Change the individual settings to customize the sound. Adjust the Threshold to set the level at which the compressor kicks in. Change the Attack setting to control how fast the compressor is applied; sometimes using a slower setting can sound more natural.