The DDJ-RR accepts both a balanced (TRS) and unbalanced (TS) 1/4" input jack, supporting all dynamic microphones. Condenser-type mics are not supported as they require phantom power, which the DDJ-RR does not provide.
With your mic connected to the input on the rear of the unit, turn the MIC LEVEL dial on the front to the 12 o'clock position and ensure the master level is also up.
Within rekordbox, open the MIXER/MIC panel by clicking the box at the top of the rekordbox window.
Click the MIC ON button and adjust the EQ as needed.
Speak into the mic and you should see activity on the meter to the left of the EQ knobs: