You can make your own tremolo effect in Guitar Rig pretty easily - just hook up an LFO (modifier) to a volume pedal (dynamics). Load both effects into the rack, then drag the little control handle (cross icon to the right of where it says LFO) onto the slider on the volume pedal. Next, open the extended controls for the LFO by clicking the downward arrow on the yellow strip on the right of the module. Here you will see the modulation mapping you just made, and you can control its amplitude with the slider.
You can control the rate with the rate knob (sync if necessary) and you can also set the form of the modulation to either sine, triangle, square or random. The horizontal and vertical lines on the wave display can be used to set the start and end points of the wave you're using.
You can then adjust the volume pedal controls to taste. I know there is a dedicated tremolo effect in Guitar Rig but I find the LFO/Volume pedal method is a lot more versatile.