Depending on what programming language you are working with the steps can different slightly, the language my team works with is Java. However if you're in C++ the only thing different is downloading tool-chains.
Once you've installed eclipse, First make sure that you have Eclipse Mars 2.0. This is the version that FRC works with and suggests that we use. Also make sure that you have the right version of eclipse. They have specific environments for Java, and ones for C++ which is shown on the page that FRC provides that you were looking at.
Second, once you've opened up eclipse click on the tab that says "Help" and it will pop a drop down list and from there select "Install new software"
From here you'll be presented with a window, there should be a button that says "Add.." Click on that.
For the name box, type "FRC Plugins" and for the location box type
http://first.wpi.edu/FRC/roborio/release/eclipse/ This website should also be on the page you were looking at but I put it here for demonstration sake
Lastly, you should see the WPI Library folder appear, from here you can drop the contents of the folder down and choose your language's plugins, for java choose javas, and for C++, choose C++'s.
I know this was kind of long, but I'm pretty thorough cause I know this kind of stuff is really tough to understand, but if you have any questions you can ask me, I'll try my hardest to help
Good Luck and I Hope you have a good season!