My crystal ball tells me you're working in an Excel VSTO 2005 project. you drag the DateTimePicker from the toolbox onto a sheet. You change the one property from false to true. When you press F5 to run the project in debug mode...

What doesn't show? the DateTimePicker control? Or the checkbox? And what "dropdown property" do you mean? There is no such property in the Properties window? Do you mean arrow at the right to display the full calendar?

FWIW, when I follow the steps above, the DateTimePicker control is visible, as well as a checkbox. Clicking the checkbox to deselect disables/enables the control. Did you change any properties besides the ShowCheckbox property? Have you added any code to your project that does anything with this control? Have you added any protection to the Excel sheet?

If you try adding the control to a NEW Excel VSTO project, change the one property, then run it in debug mode do you see any different behavior?