1. Behringer Mixer

The way you can do it is like this:

1 Connect drum machine to any of the stereo channels (5/6, 7/8 or 9/10).
2 Connect a lead from the Xenyx FX send output to the Virtualizer.
3 Connect a lead from the output of the virtualizer to any remaing free channel on the Xenyx.

The FX knob on the channel you have the drum machine in sets the level of signal to go to the virtualizer. The FX send knob under the phantom power switch is the master volume for this. In the Virtualizer you will have to adjust the reverb setting so that you only get wet coming back.

The fader on the channel you have the virtualizer plugged into controls the amount of reverb coming back.

Plugging the drum machine into the RCA inputs is ok, but you cannot apply FX to this.

Alternatively, you can do your other option, i.e. plug drum machine into virtualizer, then plug virtualizer into one of the Xenyx channels. In this case, you would have to adjust the reverb setting in the virtualizer to get a desired balance of wet and dry.