If you're talking about the different eras of the Final Demo monitors, there's an object field in the "Edit Thing" window in Zone Builder called "Parameter" (renamed to "Behavior" for these monitors) that chooses which era the monitors should emulate. The README included in the PK3 has details for which era each parameter emulates.i am still wondering how to activate the specific monitor sprites
If you're talking about the actual sprites themselves, the monitors for each era (besides Final Demo-2.0) of SRB2 have their own object types, all categorized under the "Monitors" list in the "Edit Thing" window in Zone Builder. You should be able to find them there.