There are currently, by default, two grinding balls in Ender IO. The Dark Steel Ball is one, and the other is vanilla flint. These can be used in the SAG Mill to make it more efficient in various ways. The Dark Steel Ball will increases the main output to 150%, the bonus output to 200% and decreases power usage by 30%, while the cheaper flint will will increase the main output to 120%, the bonus output to 125% and decrease power usage by 15%.
Grinding balls go in the slot right next to the GUI controls, as shown below. When an operation is started, a grinding ball will be consumed. Once the bar next to the slot becomes empty again, another one will be consumed. Different grinding balls last different amounts of operations.