I guess it's not very explicit but for step 1 you want to download the "complete.zip" file. This is the entire LDraw library. If you're using Safari it will automatically unzip this for you. Look for the "ldraw" folder in your Downloads folder.
Once you've downloaded the library, download the "application only" version of Bricksmith (the complete package is out of date). Drag brick smith from your Downloads folder to the Applications folder.
I recommend you make a dedicated Lego folder in your home directory and add a link to it on the sidebar. Drag the ldraw folder from your Downloads folder to this Lego folder that you created.
Open Launchpad and run Bricksmith. When prompted for the LDraw Library path, point it to the ldraw folder.
This should get you up and running. The Bricksmith website has a tutorial on the basic of using Bricksmith to create models.
Once you've downloaded the library, download the "application only" version of Bricksmith (the complete package is out of date). Drag brick smith from your Downloads folder to the Applications folder.
I recommend you make a dedicated Lego folder in your home directory and add a link to it on the sidebar. Drag the ldraw folder from your Downloads folder to this Lego folder that you created.
Open Launchpad and run Bricksmith. When prompted for the LDraw Library path, point it to the ldraw folder.
This should get you up and running. The Bricksmith website has a tutorial on the basic of using Bricksmith to create models.