If it uses the same inputs for PC as a pop'n music controller for PS2, then I believe the right yellow button is mapped to up, while the controller may always hold left, down, and right simultaneously (though that part may not apply if it has a mode/plug for PC only). You may have to use Joy2key if it isn't registering in osu!.
My experience comes from a small PS2 pop'n controller (with an adapter) though, so I don't know exactly how the third-party Bemani ASCs typically work.
I'd imagine that playing osu!mania charts with with a pop'n music controller would be a bit difficult as well, unless the charts were designed with that kind of controller in mind.
