If you are having trouble with a USB swiper on a mac then try the following steps to ensure it is recognized as the right type of keyboard.
Open the system preferences dialog:
Click on Keyboard
Click on Change Keyboard Type in the bottom left
Click on Continue on the dialog that opens
- Swipe a credit card through your reader
The dialog will ask you to select the keyboard type. Choose ANSI (United States and others)
- Click on Done
You should now be able to swipe cards through the reader correctly.