You can use a channel cache variable in a file path on the source node. The format that you have is very close, but it looks like you have a close parenthesis at the end, rather than a close curly brace. Try this:
Also, make sure that you have the channel cache variable set to something that is part of a valid path. If you hit 'Test Path' it should substitute the current channel cache value for the entry in your path, and display if it found the path or not.