Click the dropdown menu above the search bar (usually set to "Games") and ensure is checked. Search for Torchlight II GUTS in the list and install it.
The Torchlight II GUTS editor is powerful but requires patience. Unlike modern engines (Unity/Unreal), it relies heavily on data tables and obscure property links. The best way to learn is to open an existing level from the main game (via the editor), look at how the developers set up the triggers, and copy that logic. torchlight 2 editor