To select a dotplot region activate the Select tool:
hold down the left mouse button and drag the mouse cursor over the dotplot.
When you select a region on a dotplot the corresponding region is also selected in other Sequence View areas (Sequence details view, Sequence zoom view, etc.).
The opposite is true as well: if you select a region in a Sequence View area, the corresponding region is also selected in the dotplot view.
To zoom to the region selected click the Zoom in on the left.