I'm in the planning stages of a 4x60 building myself and wanted software to generate floorplans, which would allow me to quickly rearrange as necessary. I'm using a free evaluation version of Easy Blue Print from
http://ezblueprint.com/ to do my initial layout. While I'm no computer dummy, I found that CAD programs and even Sketchup had a steep learning curve that I wasn't willing to invest in for one project.
The trial version has limitations, such as being able to only add a 9' wide overhead door, the Pro version lets you specify the door width. My workaround for these limitations is to do the basic floorplan with the software and then do a screen capture and do further edits in Photoshop, since I've been using it for years and am very comfortable using it. I place each element (furniture, windows/doors, etc.) is on it's own layer, so I can quickly rearrange everything and scale/rotate as necessary.
I could spend the $35 to get the Pro version to give me increased functionality, but I'm happy with how things are progressing.
Granted, it's only a floorplan layout program, so 3D is off the table, but it might be something for you to look into if your needs are more basic.
EDIT: I missed that you were needing a Mac-compatible program. Sorry, this one's a Windows program only.