If your vehicle is the facelift W204, it will be equipped with COMAND Online NTG4.5.
The pre-facelift model included NTG4, which had a big PCMCIA slot and a pop-up display.
The most recent maps data is version 20.0.
The map data may be downloaded at no cost from MB, thereafter transferred to a USB drive, and uploaded to the COMAND unit in the vehicle via the USB connector located inside the center armrest. However, it is a 50GB download, necessitating a robust Internet connection.
You will thereafter be required to enter a PIN number, which is specific to your VIN; this PIN code may be acquired via MB dealers or other internet vendors. You may also get a copy of the maps data on a USB drive from them, should you choose not to download it for free at home.