I can't create a new library with it.

When you start the Library Editor you already are in a new library. It just doesn't have any models in it and it is not saved with a name. You add models to it and then you save the library with the name you want. If you already have a library open the New button should close the current library and start a new empty library.
You can add models using the toolbar button but you can also drag .obj or .zip files with models into the editor window. You can select multiple models in one go and they import very fast.
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Will I have to enter this code every time I want to use it?

I should be possible to create a command file to start the Editor and create a shortcut to it. The Furniture Library Editor forum post offers an icon to use for the shortcut.
I have no idea how that is done on Windows. The command file should probably have something like mentioned in the forum post:
You can use the same method for the Texture Library Editor .