The M-Audio M-Track is a computer interface to allow you to input audio and instrument signals into your computer.
The Digitech RP500 is a multi-effects processor for guitar - which also provides for input of the processed signal into your computer.
The Digitech RP500 provides for the output to be summed as a mono signal and sent to a guitar amplifier, or the stereo signal can be sent to a full spectrum PA or monitoring system (ideally through a mixer) or the signal can be sent to your computer via the USB output and recorded into any number of DAW software programs.
You don't need to use the M-Audio interface with the Digitech RP-500 because it can input the processed signal into your computer directly. Running the signal through the M-Audio first could potentially introduce extra latency since the M-Audio must process the already processed signal before sending it on to the computer.
For the application you described - connect everything as follows:
Plug guitar cable from guitar - into the instrument input jack on rear of RP-500.
Using an XLR cable - plug your left monitor into the left output and your right monitor into the right output. If using a mixer you will plug into either the left and right input of a stereo channel or pan one channel hard right and the other hard left. If your monitor speakers only have 1/4" inputs, use the 1/4" outputs.
Connect the RP-500 to your computer using the USB jack.
Before turning on any power - set the volume controls on your speakers and on the RP-500 to zero. Then gradually increase the volume.
You can also monitor the output of the RP-500 through headphones (jack on rear of unit).
If you were going to run the signal from the RP-500 through the M-Audio on the way to the computer, (not necessary) I would suggest using a TRS cable to make the connection. If you only use the left output on the RP-500 - it will sum the signal into mono. If you use both outputs (two cables) it will send a stereo signal. If you are connecting to a single source input such as a single guitar amp or single monitor speaker, you will need to use just one cable plugged into the left output jack.
If you have the M-Audio M-track device that has a stereo/mono switch, you can feed the left output from the RP-500 into the left channel of the M-Audio and the right output into the right channel. If your M-track does not have a stereo feature (some models) you will be sending a mono signal only so use only one cable from the left output of the M-Audio. When using the 1/4" outputs from the RP-500 you must select either the amp mode or the mixer mode using the switch beside those output jacks. The amp/mixer switch will affect the gain of the signal being output.
Using a Y cable will not give you good results because the circuitry inside the RP-500 is set up to recognize which output jacks are active and if only one cable is plugged into the left output only - it automatically switches the output to summed mono.
In summary, the M-Audio interface is not required for the application you described nor is it recommended. To use a non electronic analogy, if you want to pour salt out of a 5 pound bag into a salt shaker, you could use a funnel to direct the salt into the salt shaker. But one funnel is all you need. If you poured the salt into one funnel to pour salt into a second funnel - you still get the salt into the salt shaker - but you don't gain anything by using two funnels. If anything it slows the process.