Engage the mast track release pedal and move the mast track car back (or forward) to the middle of the mast track.
I believe the little black ball on the shock cord will then be out of the way allowing you to pull the mast foot
lock tab up so the mast base can lock in.
I hope you received the mast foot as well as this is a kinda special item that works great on Starboards, but is not compatble with other boards and other brands of mast foot pins do not work well in the Phantom.
When you pull the tab up you should see the wire clip that holds the mast foot in the car move to the open positon.