The code below should work. Each OPTION in each case is the name of your drop downs for the combobox. You place the code in format under custom keystroke. Everytime you choose one of the dropdowns it will run whatver code you placed for the case. this.getField("YOURFIELDNAME").display = display.hidden should work for hiding, replace hidden with visible and voila.
Thank you for your guidance. I have the problem solved.