I am assuming you mean the little black knob on the speed control has broken off. I will post the solution for that problem.
The speed control lever on KitchenAid mixers has a little black knob on the end (in some cases this may be white or gray). The knob is pressed onto the lever, so if the knob breaks off your only option is to replace the whole lever.
KitchenAid has a kit that will include all you need to replace the speed control lever. It even comes with instructions. Here is a link to the speed control lever kit on our website: http://www.mendingshed.com/kaleverkit.html
The lever kit includes the speed control lever (with knob), speed link, spring, instructions and a gearbox gasket.
The instructions will only walk you through how to hook up the speed control lever, so if you also need a little guidance in taking the mixer apart, and reassembling it please use the KitchenAid service manual. The manual will walk you through all of the repairs you may need on your mixer, including how to take the mixer apart and put it back together. Here is a link to the service manual: http://lib.store.yahoo.net/lib/mendingshed/kitchenaidservicemanual.pdf
Remember, The Mending Shed is a Authorized KitchenAid service center, we can help you with any repair needs for your KitchenAid appliances, plus all of the parts we carry are genuine KitchenAid brand.