If you are in a hurry, you can download all-in-one packages for Windows and UNIX from here. These include Eclipse, RTSystemEditor, RTCBuilder, and all required Eclipse plugins. It is possible to install it very easily.
The Java Development Kit (JDK) 6 is necessary. It must be installed before Eclipse. The compressed Eclipse archive is 100MB or more. Extract its contents into the directory you wish to install Eclipse into. There is no installer for Eclipse. Start it by running the eclipse.exe executable on Windows, or the eclipse binary on UNIX.