In order to make a pulsing sound in MainStage, this can be approached in a few ways: either by using an LFO to modulate volume or filter cutoff- or by using the arpeggiator to create the pulsing arp.
Using the arpeggiator is a bit simpler:
- Find a pad that has a similar character to what you're after, and then add an instance of the the Arpeggiator Midi FX plugin.
- You could set the rate of the pulse timing (either 1/16 or 1/8 depending on your patches tempo).
- In the Pattern tab, go into the Grid mode, select the little chord icon so that the arp plays the full chord, and set the range to just that chord.
- Then, go into the options tab and adjust the length of the notes.
Here is a video that covers the Arpeggiator in depth: