It comes with a hub.
Here's how to get USB instruments to work.
If you're using an SD/USB loader you need Hermes's cios. It comes with uLoader.
Install his cios 222 to ios 36 and cios 223 to ios 36 merge 37, then boot the game in uLoader with cios 223. DLC won't be compatible. (Although it won't be compatible with PAL shop channel anyway, so you'd need to change the region of that first anyway)
If you're booting from a disc, there's two ways:
1. Follow Mrkinator's Rockband Softchip guide to install cios 232 merged from ios37 and run that in softchip. Not compatible with DLC.
2. Compatible with DLC! Look for "ciosbugfix37.wad", it's in the softmii 3.0 package or you could just google it and find it elsewhere on its own; it's the only one you need. Install that and then run softchip and choose the ios to boot from as ios 37.