LyX doesn't know anything about LaTeX .cls files. You need to make the beamer.layout file available to LyX by copying it to${USER_LYX_DIR}/layouts/beamer.layout and reconfiguring LyX. USER_LYX_DIR is presumably C:\Documents and Settings\Samar\Application Data\lyx or something similar. On my linux machine (runs teTeX), this file is to be found at /usr/local/share/texmf/tex/latex/beamer/lyx/layouts/beamer.layout It'll be somewhere equivalent under MiKTeX. If not, you can grab it from the latex-beamer.sf.net viewcvs tree:
Out of curiosity really, but if lyx knows where miktex or equivalent is why doesn't it find these layout ifles automatically - or is this particular to beamer.
