There are actually two paths to Downtown Disney. One path, as already mentioned, is by Congress Park. This path enters Downtown Disney on the Westside.
The other path is close to the ferry dock and lobby. The path goes over a small bridge heading towards the LBV golf course. It enters Downtown Disney on the Westside right by Cirque. When I was there in May they were in the process of installing lighting on this path.
It might help to think of a large circular walkway around the SSR/DTD lake. Starts a SSR CP, goes to the DTD bus stop, wends through DTD, PI, WE, then goes towards OKW, then cross the bridge at the SSR Carriage House then back to CP.