Maybe you were not able to find the file because your computer is set to not show hidden files and folders. First allow hidden files and folders (from my computer, click tools and then folder options. Click the view tab. Scroll down to the option “Hidden files and folders”. Click the “show hidden files and folders” option. Click OK)
Then, try searching the file again, or go to ‘C:\Documents and Settings\<user>\Local Settings\Application Data\Microsoft\Outlook’ (this could be different depending on the OS version).
Make sure Outlook is not running. Rename the extend.dat file (to any other name). Restart outlook.
There is a Microsoft article with a similar solution here.