Edit->Reconfigure restart lyx ?
No success.
run texhash? if yes, then run
/usr/local/share/lyx/configure
and look for lines like
+checking for document class IEEEtran [IEEEtran]... yes +checking for document class aa [aa]... no +checking for document class aapaper [aa]... no +checking for document class aastex [aastex]... yes +checking for document class agums [aguplus,agums.sty]... no ...
is beamer found?
Herbert