Didn't know exactly where to post this suggestion for your manual.

On the bottom of your build/preview page (http://texlipse.sourceforge.net/manual/build.html), the solution proposed is quite complicated whene there seem to be an easier one to try first :
Eclipse preferences -> Texclipse -> Viewer settings
add new configuration named for instance MacPreview
with command /usr/bin/open
and arguments -a "/Applications/Preview.app/" %file
(check input file format is actually pdf)