Replicators will have a number of triangular pads associated with them, referred to as "switches." Usually, to activate the replicator, you need to have units on all relevant switches SIMULTANEOUSLY. You can tell the replicator's been activated by the "revving up" noise that it makes, and by the lights that activate along its edges.

Never, ever send a unit to stand on a switch unless you have first secured the replicator for that switch - otherwise you never know who might be using the replicator if you activate it! Just thought I'd mention that.
Anyway, once the replicator is active, just select your units and then tap on the replicator - You'll see what it does!
In other words, I'm not going to spoil it for you!
