Press once on the Scroll Lock key on the keyboard. It sometimes appears as "ScrLk." It is located on the top row of your keyboard to the right of the function keys.
If your keyboard doesn't have the Scroll Lock key and you are using a Windows computer, click on the Start button. Choose All Programs, and then open the Accessories folder. Click on Ease of Access, and then choose On-Screen Keyboard.
When the On-Screen Keyboard appears, click on the Slk button. It is located in the top row.