Selected item on dynamic panel

I am writing a simple math program.  I have a button for ADD, SUBTRACTION, etc.  Click on a button and I create a dynamic panel for each area.  On the new panel I have a combo box to select a starting point.  I want to display that selection in a text box to create the upper number of a math problem.  The selected item always comes up empty.  How do I get the selected value into a variable so I can use it in the program?