I think what you are looking for is a Screentip, not data validation.
First, remove the custom data validation that you inserted.
Then, double-click on the textbox control, go to the Advanced tab and type your message into the Screentip field there. This will show up whenever the user hovers over the field, but will not show an annoying red asterick or block form submission.
Hope this helps!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge