I would like to have the value of a drop down list box drive the value of a text box. What formula can I use in the rule to get the drop down list box value in the text box?
No need to use a rule. If you just want to show the value of the dropdown box in the text box, just bind the text box to the same field as the dropdown box (you can do this by right-clicking a text box and selecting "Change Binding..." or by dragging the field onto your form.
I did change the binding, but get a "i" saying that the text box needs to be in same section/group; it isn't.
I would like the dropdown box value to appear in the text box; then user can add text to it. Will your approach work for that?
If you want to use the dropdown selection as a starting value for the text box's contents, you would probably want to have the text box bound to its own field (and not the dropdown box's field), and set the text box's default value (Data tab in the properties dialog) to be the field to which the dropdown box is bound.