External Microphone Problem

I hope someone can help me. I am new to MACs. I am using Garageband 3 for the first time. I bought an external condenser microphone with a USB connection that is compatible according to the instructions. I went to Apple/preferences/input and changed the settings from the internal to the new microphone(Samson CO1U). I tapped on the Mic (far from the internal mic) to make sure it worked and it did.

For some reason (although I have gone back several times to make sure the settings remained correct) garageband only will recognize the internal mic. I tested it by going out of the room with the USB external mic and garageband still only picks up the internal mic (It only picked up the door opeing and closing but not me talking when I was out of the room). Again, I know the external mic works and is compatable because of my tests.

To further confirm that this is the case, I changed the setting to the line in port. Since nothing is plugged in there, I would expect that there would be no sound and yet the internal mic still picks up the sound.

Is there something I need to do with Garageband to recognize the external mic?

Help.... Rob