Nikon d3100 not recognised by macbook pro

Connect the camera and wait some time, then do a Apple menu > About this Mac > More information and scroll down to the USB devices and see if your camera hardware appears.

If it does, then it's a issue with software probably, is there software that might need a update to work with the camera?

If it worked before and iPhoto opened, you might need restablish that "connection"

Download and install the free RCDefaulyApp System Preference pane to see if that will help set back up the association between the hardware and iPhoto again.

http://www.rubicode.com/Software/RCDefaultApp/

Try the camera makers site and see how it needs to connect to a computer.

It might need software installed. Driver update.

You could also have a bad port or cable, try another.

Sometimes the port on the camera gets worn out, see if it connects to another computer ok or not.

Sometimes a bad battery in the camera could be responsible, drawing too much current perhaps.

Also it might be there isn't enough power to the computers USB port, connect the power or try a powered USB hub. or another port or disconnect other devices.

Perform a hardware check

http://support.apple.com/kb/ht1509

Then see if resetting the SMC/PRAM resolve your issue

http://support.apple.com/kb/HT3964

https://support.apple.com/kb/HT1379

just keep hitting at it and narrowing down the possibilities. 😀