How to get it on: Tour the local military base with a romantic partner.
Once inside, click on the building for the option to appear.
But maybe thats why its so awesome - the idea of two Sims so desperate to Woo Hoo they dont care where they get it on, as long as they can find a place to be alone. You need a Sim with a high inventing skill; just make your inventor Sim continuously invent new products until the Time Machine appears as a result of his or her hard work.
The rest is easy, provided you have two romantically involved Sims nearby: just click on the Time Machine, choose Woo Hoo, and watch the magic (or, rather, science) happen. First of all, Woo Hooing in a Time Machine means your Sims can time travel, and thats awesome.
Its already dark, so the mood is set, and compared to some of the other wild places your Sims have been getting it on around town, the theater is downright tame.