Add a multi-selection list box
To give users the ability to select more than one value from a list box, use a Multiple-Selection List Box. Place the cursor in the form where you want the multi-selection list and then go to the Controls Task Pane (View > Design Tasks > Controls). Under the Repeating and Optional section, click Multiple-Selection List Box.
Configure the multi-select list box as you would a standard List Box. If you want to give users the ability to enter their own values, be sure to select Allow users to enter custom values.