This is a multi-step process.

Mac OS X 10.8 (Mountain Lion)

Latex class stuff

Step 1: Figure out where to put your class files. (See LyX#Layouts_vs._Classes for more information.)

On Mac OS X Mountain Lion, with TeXLive (via MacTeX, which is a re-bundled version of TeXLive for Mac), you need to run:

$ mkdir -p ~/Library/texmf/tex/latex/lyx

Step 2: Put your class files in that directory.

Step 3: Figure out where to put your LyX layout file (which corresponds to the LaTeX style).

Step 4: Create a layout file.

Step 5: Put your LyX layout file in your LyX layout directory

Step 6: Reconfigure Lyx