Outlook (unfortunately) with all its different versions and service packs sometimes has problems like this. If you are using Outlook XP/2003 and it has disabled the AddIn, start Outlook and reactivate InsertText by doing the following:

If you are using Outlook 2002 and 2003:

Select Help> About Microsoft Outlook
Click Disable Items button,

select InsertText

click Enable.

If you are using Outlook 2007:

Start Outlook -> Go to inbox
Select Help -> Click Disable Items
On the screen that appears, select InsertText
Click Enable