An unmodified mouse click or drag will select one sample, one probe, one well, one cell of a heatmap, or one row of a table.
A control-mouse click or drag will add to the selection.
An alt-shifted mouse click or drag will remove from the selection.
Cmd-mouse is not the same as Ctrl-mouse. Ctrl-mouse is what is needed for making selections.
Cmd-mouse is treated the same as right-button with no keyboard event (because an Apple mouse only has one button, right-click has to be faked with Cmd-mouse).
Right-clicking an area of the heatmap will select the probes and samples falling in the area.
It is possible, for example, to use the heatmap pane to select all the probes in one cluster, and then in the probe table to use that same selection as the basis for normalization.
In a sample pair plot, selecting data points will highlight the corresponding probes.