All nodes (i.e., wires) that you wish to view in the waveform editor must have a label. To label a wire, first select the wire by clicking the left mouse button on it so that it is highlighted, and then type in the label name. If the label is moved away from the wire, it may cease to be a label. To see if a label is valid, select the wire. If the label is still valid, it will have a highlighted box around it.
Buses are basically a lot of wires bunched together. The format for a bus label is BUSNAME[i..j], where i and j are integers. Some examples are DATA[0..7] or ADDRESS[31..0]. To label a bus, select the bus by clicking on it with the left mouse button, and type in the desired label.
A wire connected to the bus must have a label which matches the label of the bus. For example, a wire connected to the bus DATA[0..7] can be labeled DATA0, DATA1, DATA2, etc., but cannot be labeled D2.