Cant Set Up My External Microphone on my iMac

im having a problem setting up my MXL studio 1 microphone on my iMac. i plug it in and the light on the microphone is lit up but when i go to my sound preferences it doesn't give me the option to select anything else but the built in mic. I dont know if it because of the version of OSX i have (v 10.9.2) or if im not doing something right. i have the microphone running into the back of my computer into one of the usb ports. im not sure if i need another piece to plug in or what but its a usb microphone. i really need help with this because its a source of my income. PLEASE HELP ME!!!!!!!