how come my android devices are not visible on my Mac? I'm new to android development so apologies if this is a stupid question.

I'm having trouble seeing my android devices in Eclipse.  I've switched on USB debugging, configured them to connect as a camera not mass storage, but when I run ./adb devices, there's no device showing, even though I have a Samsung Galaxy and a LG Nexus connected via USB.  On both phones I get a notification saying "USB debugging connected" and "connected as a camera" so the phone obviously thinks it's connected.

The problem appears to be intermittent.  Sometimes the Galaxy shows up:

Macbook:~/workspace/android-sdks/platform-tools ./adb devices
List of devices attached 
3334AB246B5300EC    device   


Then I unplug and reconnect it, and it disappears.

I've tried 3 cables and 2 devices now.  Should I just give up and use an emulator instead?
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