Step-by-Step
Macintosh - Turn off the Scroll Lock feature by pressing the Shift-F14 keys.
Note: Depending on the type of keyboard that you are using, the key combination may differ and you might need to use the CONTROL key, the OPTION key, or the 
key instead of the SHIFT key. If you are using a MacBook, you might need to attach a USB keyboard to use the F14 key combination.
Windows - Turn off the Scroll Lock feature by pressing the Shift-Scroll Lock (ScrLk) keys.
Alternately, use the On-Screen Keyboard and click the ScrLk button in the lower right. Click Start or tap the Windows key and type osk then click <enter> to open the On-Screen Keyboard.
Addendum: Right-click Excel's bottom status bar and put a check beside Scroll Lock to get a scroll lock indicator on the left side of the status bar.
Cause
When the Scroll Lock key is "on", the arrow keys on an extended keyboard can no longer be used to navigate between cells in an Excel spreadsheet.