It is best to have a 3-button mouse in Squeak, and I suppose you have one. There are various ways to refer to the buttons of the mouse, but I will use the most natural one: Left-Click, Middle-Click, Right-Click. These you all need to work in Squeak, as will be shown in the next point. (In some cases Clicking with shift impressed will also work.)
If you have a 2-button mouse, MiddleClick = Cmd LeftClick. (And Cmd = Alt on Windows keyboards.)