You have to learn Black Ops which you do via the Headquarters building. That's the building you start with, however there are other buildings that need to go up before you can learn Black Ops.
- Select the Private Military tab.
- Build a Barracks
- Build a Vehicles Bay (needed to get access to the Prison upgrade)
- Learn Prison Module via the Barracks
- Click Base Quick Selection Button
- Click Black Ops button to learn
- Click Black Ops Tab to access the top tier units