You need to have a form with an underlying table with a field that the selected results from the combobox will go (I assume you already do).
In order to retrieve the existing records for the combobox, I would suggest creating a query and basing the combobox on the query.
With the form in design mode, use the combobox wizard. It will ask you to chose the table or query where the existing records are coming from. Chose the query that you made. When it ask you which field you want to save the data in, pick the field in the underlying table.
If you want to be able to add new things to the combobox from your form, make sure the Limit to List setting in the combobox properties is set to No.
Hope this helps. (And I hope I have understood your problem)