There may already be a way to do this, but I can't seem to figure it out.
If I right click an .nxlv file and choose "Open with" then select the NLEditor, it opens the editor but doesn't load the level. ???
I'll see what I can do.
Also, could it be possible to open levels via drag & drop into the Editor?
Both of these implemented, in commits 9e446a0 (Open With or drag-dropping onto the EXE) and ddfe8c3 (drag-dropping onto a running editor instance).