If you search the forums for "STI" and "handlebar bag" (picking the right query words is always a bit of a crapshoot, but these found some good hits for me), you should be able to find some of the previous discussions. Basically, you need to use a bit of hardware to reroute your cables. There is an excellent little article at
http://www.arkel-od.com/technical/shiftersBB.asp, which discusses what little bits of hardware work.