I'm glad you found a solution for your scenario.
To select a value into a combo box you should just set the property used by the related combo box to the desired value. For instance if you have a combo box (COMBO_PROP) with two items:
Value: value2 (selected by default)
and you want the "value1" be selected in the combo box you can use a "Set installer property value" control event which will set the "COMBO_PROP" property to the "value1" value.
Hope this helped too.
All the best,