Yes, you do need Secret Ops - Unknown Enemy is an
add-on, not a stand-alone game.
You need to do the following things:
1. Uninstall Unknown Enemy - you'll need to reinstall it after you've installed Secret Ops.
2. Download Secret Ops
here, and install it.
3. Install Unknown Enemy in the
exact same location where you installed Secret Ops. They have to be in the same folder - otherwise, UE will not work.