What do you mean by "enter the Outlook calendar"? Enter a new appointment in the calendar? Display the Calendar folder? In a new window or in place of the folder currently displayed?
FYI, there is a newsgroup specifically for general Outlook programming issues "down the hall" at microsoft.public.outlook.program_vba or, via web interface, at http://www.microsoft.com/office/community/en-us/default.mspx?dg=microsoft.public
.outlook.program_vba

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Sue Mosher, Outlook MVP
Author of
Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
> My boss would like me to add a button so that he can enter the Outlook
> calendar with one click when he is writing a mail.
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> I woudl be very happy if anyone could help me on this.