There are two checkboxes in Excel.
One is from the Forms toolbar and the other is from the Control Toolbox toolbar.
If your checkbox is from the Forms toolbar, then rightclick on the checkbox and
choose "Format Control".
Then on the Control tab, type in the address of this cell.
If your checkbox is from the Control toolbox toolbar, then go into Design mode
(another icon on that toolbar), rightclick on the checkbox, choose Properties
and look for LinkedCell. Type in the address for that cell.
Make sure you leave design mode when you're done making your change.