I can't change the contents of a combobox

. I use what is specified in the help files :
MyComboBox.List(0, 0) = "Center"
MyComboBox.List(1, 0) = "Center"
MyComboBox.List(2, 0) = "Under"
but nothing ... Let me specify here that I am trying to modify the existing contents of the combobox (not to add or delete ones) .