Quote from PhanxIf that's your whole code, you have several major problems:
1. The frame reference, event name, and event arguments are NOT set as global variables since WoW 4.0 (and it's been recommended not to use them since WoW 2.0). One consequence of this is that there is no global "this". You should be seeing several errors in the vein of "attempt to index global 'this' (a nil value)".
2. You don't have a frame, so you have nothing to call the :RegisterEvent or :SetScript methods on.
i'm guessing there's probably an additional xml file that calls HelloWorld from a frame OnLoad script.
that would explain the global namespace function and the use of "this:" as a reference.