The most likely explanation is that the Beamer LaTeX package is not installed. Since you're on Win 7, you are almost surely using MiKTeX as your LaTeX distribution. Run the MiKTeX package manager and look for the Beamer entry. If it's not installed, tell the package manager to install it. Once it's installed, start LyX, reconfigure (Tools menu), and then restart LyX. You should be able to create Beamer docs.
Paul
