SIMPLE -- I'm talking WAAAAY Simple 3D program for Mac without being an engineer I do "barn hunt" --- a fun dog sport.  What isn't fun about it is creating courses.  I need something really, really, really, really simple... think playing with LEGOS simple.  I need something that I can set the scale for a 37' x 24' space and add hay bale dimensions and stack them.  We have multiple levels of hay bales to stack- up to 4 in some places.  

All I need is to take my bales (rectangles) and have 4 separate colors and stack them as needed.  I don't need to build the Mayan ruins.  I don't need to model characters and add animation.... I just freakin' need something easy that doesn't require me to have a degree in engineering and architecture.  Is there ANYTHING that can do that??????
123D is a free super simplified version of a software I used called Autodesk Inventor it is a basic 3d design program that can get sort of complex the farther you dig but the interface is simplistic and you should be able to find tutorials on sites like Youtube. As Jamie said Lego Digital Designer might also be a good match.