How do I open the GUI?
The hotkey is Control + T by default. There is also a button on the shortcut bar that says LTN.

The GUI won't open. It says something about translations...
The mod translates the names of every material (fluid and item) before it lets you open the GUI, to allow for localized search. Sometimes the library will slip up if you save and reload while it's translating. If this happens, run the /LtnManager refresh-player-data command, and the GUI should become available shortly.