1. Make sure that the insertion point isn't inside the textbox... you should have it selected, not be inside it, so click on the border to select. Since it works consistently on my Office 7 machine, I think this is the issue you are having.

2. For textboxes that you made using the Shapes button, add them to the QuickParts gallery instead of the textbox gallery. For ones I create using "draw textbox", I can add to the Textbox gallery without problem.