There are two ways to select multiple shots
1. If there is an empty slot adjacent to the shots you want to select, press the left mouse button over the empty slot, then, holding the mouse button down, move the mouse pointer over the shots you want to select.
2. If there is no adjacent empty slot, hold down the Ctrl key while you move the mouse over the shots you want to select.
Whichever method you use, moving the mouse over un-selected shots selects them and moving it back over selected shots deselects them.