There is no Solver for non linear problems in LibreOffice 3.4 beta5.
It seems to be completely missing from the builds.
Open the extension manager and look for "Solver for nonlinear Programming".
Fridrich solved this by http://cgit.freedesktop.org/libreoffice/bootstrap/commit/?h=libreoffice-3-4&id=c2f4991a21caf753e9c71c62be29333147ee0acb
Hmm, the extension is installed and registered in the last build. Though, I do not see the two non-linear solvers in "Tools/Solver/Options/Solver Engine". It needs more love => reopening.
seems ok in LibO 3.4.0 and 3.4.1rc1 (I see the plugins in solver options)