how come my android devices are not visible on my Mac?
I know it's too late. but I just had the same problem. I tried to connect my android phone LG Google Nexus 4 with my Macbook pro. 

In USB computer connection only "Camera (PTP)" was working but Connect as Media device (MTP) to copy files from Android phone's memory nothing was visible in the mac.

And I got it solved from the following URL
http://www.android.com/filetransfer/
provided by the android.com