The FLUX library comprising of the kernel program and constraint handling rules is available here:
FLUX library 3.1 (Eclipse) : .tgz archive (for Unix), .zip archive (for Windows)
   In FLUX 3.1 a bug from release 3.0 has been fixed.
FLUX library 2.0 (Sicstus) : .tgz archive (for Unix), .zip archive (for Windows)

Please consult the journal paper FLUX: A Logic Programming Method for Reasoning Agents (2004) for an introduction to the Fluent Calculus and the FLUX programming system, available here: PDF (336 KB), or PS.GZ (164 KB).

For an introduction to the Constraints that are new in FLUX 3, please consult the conference paper Handling Implication and Universal Quantification Constraints in FLUX (2005), available here: PDF (235 KB), or PS.GZ (79 KB).

Here you can find a short comparison between FLUX in Eclipse and FLUX in Sicstus: PDF (25 KB), or PS.GZ (13 KB) and some hints for converting examples between these systems: PDF (51 KB), or PS.GZ (23 KB)

You may also find the example below useful. It implements the so-called Wumpus World using FLUX.
Wumpus World (Eclipse) : agent, simulator
Wumpus World (Sicstus) : agent, simulator