There are two ways to select multiple clips.
1. If there is an empty cell adjacent to the clips you want to select, press the left mouse button over the empty cell, then, holding the mouse button down, move the mouse pointer over the clips you want to select.
2. If there is no adjacent empty cell, hold down the Shift key while you move the mouse over the clips you want to select.
Whichever method you use, moving the mouse over un-selected clips selects them and moving it back over selected clips deselects them.
From Version 2, if you have selected "Arrow keys and Enter" in Options View (n) on the Tools menu, you can also multi select by holding down the Shift key and moving the highlight with the arrow keys.