It's the compression tab. That's not a naïve question at all because it doesn't say "Limiter" it says "Compression." A limiter is a type of compressor.
Go through all the presets. Do not pay any attention to the names. Sometimes, a user will think "Oh, I can't use a hip-hop preset on my acoustic folk song!" but that's not true. Those are just names, so go through each preset and see which one comes as close as possible to the sound you are looking for. Almost always, you will want a multiband compressor or limiter. Once you settle on a preset, make sure the Makeup gain is ON. Make small moves. 1 or 2 dB is a lot in Mastering.
One drawback of the Mastering section is that you have to manually turn off the EQ and Compression when listening to the finished Mastered track or else it will double-process. Make small adjustments based on your ears and redo the Mastering operation. Rinse and repeat until you get what you want. If it's not loud enough (use the meters) then add more gain.
With the mulitband compressor, the gain controls affect each band individually. So they act as a spectral adjustment. Adding or reducing the gain in a band will also increase or decrease the energy in that band. If you need more midrange, you can touch up the midrange band gain.
Don't go nuts on it though. In Mastering a little goes a long way. You should already have a good sounding mix to begin with. Mastering is a subtle polishing operation that give you a deliverable product. It's not supposed to fix a poor mix.