Hi,
To add coordinates to your GPS you first of all need to know the Grid reference you require then tap Mark Waypoints (on main menu screen) this will take you to a screen that says Save or Save and Edit tap Save and Edit this will tale you to another screen select Change Location this will take you to another screen at which time you can tap in your coordinates, don't forget to tap the green arrow bottom right to complete the operation.
If you need further assistance take a look at
http://www.gpstraining.co.uk