that means you already had it installed. it’s a weird process the way it works and I don’t know why the creator distributed the click.once application instead of the .exe for it. go into remove programs, find rb3maker, uninstall, then try launching it again.
You are about to leave Rhythm Gaming World and access the external songs DB. While no song or file sold for commercially available videogames is hosted there and only fan generated content is allowed, Rhythm Gaming World has no direct control over the database and no files are hosted here. Alright, we had to made you aware of this, now you can visit the database!