In the Preferences tab, check the option Allow Left Click Script.
Then go to the Global Lua tab and see if you already have a function named sp_left_mouse_up(x, y, fwKeys). If you do, just add your call to TabTip in there. If not, simply add the function. Here's what I used for testing and it seemed to work fine for me:
function sp_left_mouse_up(x, y, fwKeys)
if acGetMouseCursorType() == "IBEAM" then
acDisplayBalloonTip("IBEAM", "Hello!", 1) --display a balloon tip