In previous versions of Outlook, most forms had a field on the lower right called Contacts which you could use to link a contact to an item which would then show up on the Activities tab. You either used this feature, or you didn't and for those who use it, Outlook 2007 hides it by default.
To enable it, go to Tools, Options, Contacts Options button and add a check to the option under Contact linking to show it on forms.
Update: This applies to Outlook 2010 as well. To enable linking, you'll go to File, Options, Contacts and add a check to the option under Contact linking.
As of Outlook 2013, the contacts field is not visible on forms and can't be enabled in Options. It is possible to enable it using a registry change. See Show Contact Linking fields in Outlook 2013 for more information and ready-to-use reg file.
Published September 21, 2006. Last updated on October 30, 2013.