By default Cubase will have routed the selected channel to the FX bus, however you need to activate the send. Expand the "Sends" tab in the mixer on the channel you created a send for. Turn on the listed send in the inserts (it'll be greyed out) and it'll then turn light blue, you can adjust how much of the signal is sent to the fx bus.
You can also create new sends by clicking on an empty insert in the sends tab. Then selecting where you want the signal to go.
P.s if you click the speaker icon on the send you can change between pre and post.