The lever underneath the trunk-had to look at a parts fiche-is to release the trunk from the mounting.
Not sure how you got that from the diagram, but the lever is to open the trunk. To remove it from the bike requires removal of 6 small bolts from the inside of the trunk.
Stating the obvious, but is it locked?
You can remove the key with it locked (vertical?) or unlocked (horizontal).
Try both ways. The handle only works in the unlocked position and feels completely loose and disconnected when locked.
As I recall, vertical key, turn it left, trunk opens.
Turn it right and remove it is unlocked and restores the handle function.
Earl raises a good point and is correct about its standard operation. If it is unlocked, you can remove the taillights (the two screws from inside right behind each) and see if the cable has slipped off its holder or something. If you have small enough hands, you could possibly even reach through the taillight opening to work on it. But if you have to replace the latch or can't fit your hands through the taillight holes, you must disassemble the ENTIRE trunk, OFF the bike. I just had mine apart yesterday. The service manual shows how, but if you don't have one and find that's what you need to do, report back and one of us can type you through it.