First Method:
Right-click anywhere in Market Watch, then click on "Symbols.", A new window will appear that contains all the available instruments. Double-click the desired instrument. Or you can click "show symbol" to show it in Market Watch, or "Hide symbol" to hide it from Market Watch.
Second Method:
Simply, click on "click to add" as shown in the next photo:
Then you can write the name of the desired instrument, then press "Enter".