Well, you need to populate the combo and change the way the combo binds depending on your needs.  If you are using a combo as a DropDown, then you can type in the field and you will want to change the BindingProperty to Text instead of SelectedValue.  If you are using it as a DropDownList, then you will want to use Keith's suggestion and use the PopulationDataSourceSettings (or manually load the combo, whichever), populate with an Enum, or whataver avenue you want to travel.  If you use Enum or BusinessObject population of the combo, then the SelectedValue will hold the value to which you will want to bind (in most cases).  I recommend looking at the help or some of the samples...this is a very common task that is used a lot so there are lots of posts and samples on this topic.

PopulationDataSourceSettings, PopulationType, Columns, BusinessObject - they will get you started

Keith Chisarik

Using a SF Maintenance form with a business object. Dragged a SF ComboBox to the form. The BusinessObject property was already set, I just set the BindingField property to the column name in the business object. When displaying the form, the combo does not display the data. A textbox does. What other property is required.