There are two versions of EclipseFP. Version 1 is fairly old and no longer actively developed. Version 2 is still under heavy development, but is the version recommended for end use.
Development of EclipseFP 2 had been picked up by Thomas ten Cate in the context of a Google Summer of Code project. See also his project blog.
The project is now actively maintained and developed by JP Moresmau. Version 1.108.0 is the first formal release of that code. Current version is 1.109.0.
You need a Java 5.0 compliant virtual machine (JDK or JRE) installed on your system. (For Linux, Solaris and Windows systems you can find one at http://java.sun.com/).
You need Eclipse 3.5.x (Galileo). If you don't have Eclipse, download it from http://eclipse.org/downloads/. The Eclipse "IDE for Java Developers" package is sufficient.
You need a GHC installed on your system. See http://haskell.org/ghc/ for more information and for downloads.
You need Scion, the Haskell IDE library. See
http://code.google.com/p/scion-lib/
for more information.
You cannot use the version from Hackage (0.1.0.2) since commands have been added for eclipsefp. You'll need to build Scion from source
(git clone git://github.com/jpmoresmau/scion.git, runhaskell Setup.hs configure/build/install)
Download and installation of the plugins themselves is done via the built-in Update Manager in Eclipse. You can find installation instructions here.
Complete instructions to download and build Eclipse FP2 can be found here.