Hit Selection Mode > Press and hold "s" for source node and click on the node you want > Press and hold "t" for target node and click on the node you want > Onc

Hit Selection Mode > Press and hold "s" for source node and click on the node you want > Press and hold "t" for target node and click on the node you want > Once the names of the nodes appear to the left under the header "+/- Edge", hit Add Edge.