Simply turn the Monitor Mix control all the way to the Input side, where the solid white block is. At this point, the Meters switch from showing the stereo output level to showing the levels of each input.
The Mic input is on the upper meter and the DI input is on the lower meter. For a good signal level increase the gain of the input until you're getting a signal that peaks around the yellow -6dB indicator. If the signal lights up the Red 0dB indicator then you have clipped the input so simply decrease the gain of that input to minimise the risk of adding distortion to your track.