When you receive and error that Outlook can't find and add-in, the usual cause is that you uninstalled it but Outlook didn't delete the file it uses to keep track of installed add-ins. This file is extend.dat and is found in the (hidden) local app data file under your user account. It's in the same folder as your Outlook data files.
You can quickly jump to it by typing or pasting the following command in the Start Search field in Windows 7 or Vista.
Delete it and restart Outlook. Outlook will rebuild the extend.dat field and the errors should go away.
In most cases, you'll need to restart Outlook for any changes to take effect.
Tip: when you use these commands to open hidden files and folders, you can see the contents of the folder without changing your settings for Hidden files and folders.
Windows XP Location
If you are using Windows XP, extend.dat (and the data files) are at
%USERPROFILE%\Local Settings\Application Data\Microsoft\Outlook
Type or paste it into the Start menu, run command (you can also use Windows key + R to open the run command) and press Enter.
Open the Add-ins Dialog
To view the installed add-ins, go to Outlook 2007's Tools menu and select Trust Center, then Add-ins. In Outlook 2010, you'll go to File, Options, Add-ins. The list of installed (and the active or disabled state) is on this screen. Click Go at the bottom of the screen to open the dialog which will allow you to enabled or disable the add-ins.
In older versions of Outlook, go to Tools, Options, Other, Advanced options, Add-in Manager and COM Add-ins.