This is a follow up to an older thread on the same problem. Search
groups.google.com for the same subject and you'll find the previous
thread (which lacked an answer). I'm just posting here for posterity.

When sending email from Excel and expecting to see your Outlook
contacts box come up for you but instead you see a strange box letting
you choose "shared contacts" or "this identity's contacts", the answer
is that Internet Explorer is set up to use Outlook Express for email
instead of Outlook.

Open IE, go to Tools Internet Options Programs tab. Change the
email option to Microsoft Outlook and restart Excel.

Hope that helps. :)