Download & Install
Firstly, you have to download the mod. This can be done here: http://ets2mp.com/index.php?page=download
When the mod is downloaded, you have to install it. Go through the steps.
When it asks you for your game directory:
Go to steam and right click on "Euro Truck Simulator 2" on the list to the left on your game library.
Choose "Properties".
Choose "Local Files", and click on "Browse Local Files".
Copy the path on the top, inside the text box. (For example, mine is "D:\Programfiler\Steam\steamapps\common\Euro Truck Simulator 2").
Paste the path into where the installation asked you for it.
When you have finished installing the mod, you should have an icon on your desktop called "Play Euro Truck Simulator 2 Multiplayer". If not, simply search from your start menu.
Click the icon.
When the game runs, you will have to log in with your ETS2MP account. Of it doesn't accept @'s, copy one from outside the game and paste it in. You will also have to choose a server based on your location.
When you are logged in, you can choose a profile without mods, and start driving. The game will automaticly connect you to the server.
You can not play multiplayer if you start the game from steam. You have to use the multiplayer program on your computer!
Happy truckin' !