For XL2003..from menu View>Toolbars>Control Toolbox>Double click the
checkbox>click the worksheet>Click the toolbar checkbox>Close the toolbar
For XL2003..from menu View>Toolbars>Forms>Double click the checkbox>click
the worksheet>Click the toolbar checkbox>Close the toolbar
For XL2007..from the menu bar select Developer tab>then Insert and double
click the Checkbox in the Form Controls section or click and draw the box on
the sheet.
PS: If you cannot find the developer tab in 2007;
1. Click the Microsoft Office Button, and then click Excel Options
2. Click Popular, and then click Show Developer tab in the Ribbon