I successfully activated the arm / thigh plugin. But when I rez the cuffs, the "Arm" / "Thigh" words are lit up on the HUD, but the plugin cuffs are invisible. Any ideas?

Most likely either the activation/setup was borked by Second Life, or the Second Life servers corrupted your cuffs.

To fix,

Option 1:
Re-activate your arm / thigh cuffs, by wearing your Activator, and choosing "Re-activate" when asked whether you want to re-activate.

• If the same cuffs are invisible, most probably the cuffs are corrupted (you can try your luck again, or go to Option 2). If you get a different set of invisible cuffs, seems like SL is borking the setup at the moment. Retry at a later time, when SL is more stable.

Option 2:
Probably the SL servers have corrupted your cuffs. You need to get a new set, by getting your cuffs updated (even if you just bought them) at the Secret Warehouse.

This will give you a fresh new set. Then setup the cuffs again, and activate the plugin.