I had this same problem, after an initial install of WP and MAMP on my iMac.
After reading through numerous help forums, the fix was to download/install the "Use Google Libraries" plugin through your WP Plugin tab.
Once it's successfully installed, you have to actually "activate it" through the same Plugin tab. I didn't realize this at first, and thought that the problem wasn't solved, but once I activated that plugin, and refreshed my browser window, the interface was displaying correctly.
On a related note, I was having issues (as I found out, many Mac users were with the WP 3.4.1) with the Post Editor, both Visual and HTML section. The text was showing up as white in the Visual Editor, and I couldn't even access the HTML editor at all.
Installing and activating that Google Libraries plugin seems to have rectified this problem as well.
Hope this helps; I'm a total n00b using WP, so I was happy to find a solution.