You bought the Arm or Thigh Cuffs, but the box did not contain either. Instead, there were Activators.

This is because the Arm and Thigh Cuffs are plugins for the core Cuffs (i.e. the Wrists and Ankles). As spelled out on the vendor and the web, the core set is required for the plugins to work. 

There are three vendors for each type of Cuff (i.e. leather or padded-steel) - the core set, the Arm and the Thigh plugins. Once you get the core set, and set them up, you can simply wear the plugin Activators to get your Arm or Thigh cuffs.