To manually shut your trunk, place your hand on the trunk and push it down.

It will lock when you lock the doors. If you want to lock it with the key so that it will not open when the doors are opened, then insert the key in the trunk lock and turn it until the key hole is 90° of the position it is in now. I think that may be the horizontal position that locks it. if you've got the owner's manual, that will tell you for sure.

__________________
Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles