Giants Editor is actually included in your Farming Simulator 2011 game purchase. So you actually have it on your pc right now, just have to install it. So to get to the installation files do the following.
1 - Navigate to you root fs 2011 folder. Easiest way to do this is right click on your fs 2011 desktop icon, then click open file location.
2 - You should now be in the root folder of fs 2011. Now you Should see a folder called SDK open it up, inside is several files, one of which is called Giants_Editor_4.1.7
3 - run the Giants_Editor_4.1.7 application (double click on it) and the application will guide you through the rest.
ahh seederman beat me to it, dumb slow internet
this may also be of help to you once you get it installed:http://www.ls-uk.info/forum/index.php?topic=1520.0