You can find the latest documentation at WD Fusion 2.5 User Guide. Below are links to the available user guides for earlier versions of WD Fusion:
August 2015 -
WD Fusion 2.5.x User Guide
June 2015 -
WD Fusion 2.4.x User Guide
May 2015 -
WD Fusion 2.3.x User Guide
May 2015 -
WD Fusion 2.2.x User Guide
April 2015 -
WD Fusion 2.1.x User Guide