The "uHUD" button calls the uHUD (Utility HUD) menu, which add loads of fun to your LULU experience. 

If nothing happens when you click, it could mean:

• the sub is not wearing the uHUD. Look for the Utility HUD that comes in the LULU Collar folder and wear it.
• the uHUD is borked.
  - Just right click the uHUD, then
  - on the SL menu, look for "Reset all scripts" (In Firestorm, it's under Build > Scripts > Reset Scripts)