Download and install original LDraw Package
The original LDraw package contains the parts libraries needed to get started creating LDraw models. Clicking Step 1 will take you to the download page. Download "complete.zip". If you are using Safari it will automatically unzip this file for you. Look for the "ldraw" folder in your Downloads folder. This step cannot be skipped.
Bricksmith is a full-featured model editor for Mac OS X 10.3 or later. Bricksmith provides a spinnable 3D model view, a fast, comprehensive part browser, easy access to the full color palette, and inspectors to tweak part components. It also supports submodels (MPD), steps, and primitives, as well as the usual features you expect in a Mac application (undo, copying, scrolling, and drag-and-drop reordering of file contents). And it is open source.
To download, visit the Bricksmith site, click the download link at the top of the page. Note: the Bricksmith site distributes a version of Bricksmith that includes an LDraw parts library. This library is not updated by LDraw.org. It is recommended that you download the "application only" version and follow step 1 above to ensure you have the most recent update to the library.
Learning how to use your new editor may be a challenge, but with a little practice you will get the hang of it.
Introduction to LDraw Utilities
There is a lot you can do with LDraw files beyond just creating LEGO models! Many contributors have developed utilities to make the most of the LDraw file format. Check out LDraw.org's list of software packages to download which supplement the standard editors.
Need help with any of this? Ask on the forums