Add-ins are programs that you install and use with Outlook to "extend" Outlook's capabilities.
In Outlook 2010, go to File, Options, Trust Center, Trust Center Settings, Add-ins. In Outlook 2007, add-ins are listed in the Trust Center, accessed through the Tools menu.
In Outlook 2007 and older, you'll find most add-ins under COM Add-ins, while a few may be listed under Exchange Client Extensions.
Note that Exchange client Extensions are not supported in Microsoft Outlook 2010 and up.
To enable or disable add-ins, click the Go button and add or remove the checkmark in front of the add-in name. Click Ok to save the changes and return to Outlook.
To check for disabled add-ins, select Disabled Items from the dropdown and click Go. Click on each add-in that you want to enable then click the Enable button.
Outlook 2003 and older
In Outlook 2003 and older, the Add-ins dialog is at Tools, Options, Other tab, Advanced Options button.