Hi,
I think you've to get the list of email addresses from MS-Outlook Address Book using MAPI Module and add all those to the Drop Down box in the Form's FormLoad event.
Look at the following MAPI Example code in C#:
http://g8.cx/mapi/ExtendedMapiWithCSharp_sourcecode.zip_
(Admin note: the above zip file has not been virus-scanned. To download, remove the trailing underline and download at your own risk).
-Attari-