You got a fill plug and a drain plug. Remove them both, preferably the fill plug first to make sure it comes off - it likely takes a 14mm hex/allen wrench. Fill it up until it flows out the fill plug. You may need something to pump the fluid uphill. NAPA has a nice suction gun for about ten bucks that I use for the job. Replace the plug washers - may be one on each or might just be one.

__________________
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