Are you talking about the uber-replicator? It's just eye-candy, as far as the triggers are concerned. Troops cannot enter it and be duplicated or anything. It only appears in M14, and if it gets used, the planet, well, you know.
I used it in "Sword of the Sun" and made it work with what I think is a rather clumsy combination of switch/areas/triggers. Eh, it was functional, I suppose, even if it isn't the most elegant implementation.
If you're talking about a standard replicator, you can place it as any unit. To make it active, there is a "make side1 replicator active" trigger that needs to be present. You might also want to make sure that the replicator you place is for the appropriate side, as well....