You can add Serum to an instrument (MIDI) track in your DAW using the same procedure that you use with any software instrument. At this point, sending MIDI notes to Serum should trigger the default sawtooth sound.

Similarly, you should hear the sawtooth sound if you click any of the piano keys at the bottom of the Serum user interface (UI).

Note: If you are not sure how to add an instrument to a track, refer to the documentation that came with your DAW.