After opening the Editor for the first time you need to setup a few things.
First go to "Settings".
In the new Window you must setup your ROM directory, like this one:
(NOTE: Please only use games that you have dumped, DO NOT support piracy, developers put a shit-ton of work into this game!)
You can also optionally change your project directory, but it's already set to your current home directory.
The path is saved automatically, and after closing the window, your back in the Launcher.
There you must either reload (F5) or re-open the app to let the changes take effect.
After that you should be greeted with a list of Models and Shrines.
For further information see the Wiki-Entries for the Model and Shrine-Editor.