My guess is that the object Aliases.appX. appXComboBoxPopupForm.ComboBoxPopUpListBox does not support the method "ClickItem".
So... next question... what happens when you try to record the desired action? How does the code show up? I don't recommend retaining the recorded code for your actual tests but it is useful for exploration purposes.
I'm assuming that there is some other component as well that you initially click on to get the "ComboBoxPopupListBox"... does that component have the "ClickItem" method?
Finally... the trial version should support all script code types... C# script may not be available in the wizard but if you create a Project Suite first and then right click on the suite and select Add -> New Item, you'll get the following screen.
