So, there's an event you have to attach code to if you want to do something when someone changes the value of the control. Usually, you don't do that – you just let them dicker around until they hit an OK button, and then you read the values.
To read the value, it's just $box.SelectedItem.
You also have to distinguish between a ComboBox (a list of values, but the user can type their own) or a simple DropDownList (just a list of values, must pick one). You set a property of the combo box to tell it how to behave. You can do that in the designer.
SAPIEN also runs a great set of forums to help people with GUI issues – drop by there if you need specifics. I don't use the product every day, so I don't have all the answers off the top of my head.