The pin that you see in the center of the part still in the watch is a screw which screws into the pusher (assuming its not broken off)
To refit this you will need to open the case, remove the movement and screw the pin back into the pusher.
It is straight forward but best done by a watchmaker. If the pin is broken then you'll need a complete new pusher.
http://forums.watchuseek.com/f20/speedmaster-pusher-issue-3410058.htmlClick to expand...