To select an item, click the item in the Available list and click the >  button to move it to the Selected list. To select all items in the Available list, click the Select all >  button.

To deselect an item, click the item in the Selected list and click the <  button to move it to the Available list. To deselect all items in the Selected list, click the < Select all button.

You can use the Shift + click combination to choose adjacent items, or the Ctrl + click combination to choose nonadjacent items.