What is GUI?

Once again i tell you, i am a N00B, so please dont hold this against me.
I got this really cool add-on, Server Flooding, however, it requires you to do something in the GUI.

What the hell is GUI, and how do I use it?

a GUI is the box that pops up with options, like whe you press Esc in game and the box pops up, for the flooding mod you need the GUI to control how high you want to flood, waves, etc.

where do i get this GUI you speak of?

look on the BL forums

OR... Go to your options menue and click onto controls :3

Edit:Or follow kittehs thing