This is a similar question answered in the Forms forum. I will just paste the answer as I think it is the same problem.
You have to be careful replacing text boxes with combo boxes. When you view properties of the form, (View, then Properties) and click the upper left corner of the form (in design view) you should see the record source as the main form you created. The form wizard will match the control of the form to the source field. When you replace the text box with a combo box, you must have a lookup function for the combo or list box to look up. For new users, the best way to make the wizard do this is to specify a lookup field in the source table.
Go to the design view of the table you made, and insert a new field below the one you want to become a combo box. Type a name close to the one you will replace. In the type column, type lookup...
The lookup wizard will appear and you can follow that easily. The best way to generate lookup fields is to use a table. You can add or change lookup values easily in the future. Choose this table when going thru the wizard.
In datasheet view of the table, click the column header of the field you wanted to change, then CTRL C. Click the column of the new field then CTRL V. This will prevent you from loosing data in the field you are replacing. Go to design view, delete the old field and rename the new field the old field name.
Now go to your form, make sure the property for the combo or list box shows the control source the same as the field you just fixed.
If you do everything right in the source table, the auto forms and auto reports are a good way to start out, then just tweak them to your own needs as you go.
Hope this helps.