Most ActiveX controls, including MonthView, are 32-bit only . If you are running 64-bit Microsoft Office, these controls will not work. You will need to use a custom VBA-coded calendar or switch to a 32-bit installation of Office. If you're having trouble, let me know: Are you using 64-bit or 32-bit Office ? What is the exact error message you see? Which operating system are you on (Windows 10, 11, etc.)?
If the control does not appear: