You need to add a handler/procedure to your list control holding the column names. When the user clicks on this list the handler will be called and you can read the value from the column from the selection.
You should define your Dialog like this - at least. You will probably need some extra controls as well.
Control PopupMenu Width 200 ID 100
Title From Variable marrColumns
Control EditText Width 200 ID 200
This is how your handler could look:
Dim nItem As Integer
Dim aCol As Alias
'**100 is the ID of your popupmenu/list control
nItem = ReadControlValue(100)
Fetch First From Selection
'**marrColumns is a array variable holding your column names
aCol = "Selection." & marrColumns(nItem)
'**200 is the ID of your EditText control
Alter Control 200 Value aCol