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.