Haven't used keyword tests in years but try the following:
In your keyword test when you "Select object" make sure you select the entire form NOT the text box it's self.
1. Click next, then add property name "ReadOnly" value "false".
2. Add a second property like "Name" and "*thenameofyourtextbox*" (with * wild cards).
3. Click next and select the Keys method (if you make it to this point it found an object)
4. Enter the value you want to key in
5. Click Finish