Thats a short tutorial to install the favorite mods and maps in your game
First of all you have to download the mod from a specific website.
If the download is a .zip file, you just have to copy that downloaded file and paste it in this folder:

C:\Documents and Settings\...\My Documents\My Games\FarmingSimulator2011\mods

After that, you can start your game and buy the desired mod from the shop.

The same procedure its needed to install the custom maps made by modders.
Download the desired map, it should be in .zip format, and place it in the same folder:

C:\Documents and Settings\...\My Documents\My Games\FarmingSimulator2011\mods

If the downloaded mod or map is compressed in a .rar file or other compressed format, first of all you will have to unpack the archive to get the .zip files. After that, place the .zip files extracted from the archive in the folder mentioned above.

Enjoy the game using your desired mods or maps !