A family of Microsoft word processing software products for creating web, email, and print documents.
It sounds as if you have some version of Word 2010. In that version, the Insert key toggle is disabled by default. If you have a tendency to hit the Insert key unintentionally (all too easy to do when aiming for Backspace or Delete), you may want to leave it this way and just add the toggle to the status bar instead. Right-click the status bar and, on the menu, click on Overtype to put a check beside it. You will then see Overtype or Insert on the status bar; clicking on that space will toggle the function.
If you do want to enable the Insert key, however, go to File | Options | Advanced: Editing options and check the box for "Use the Insert key to control overtype mode."