Hi Justin,
Sounds like most of what you are doing sounds correct, but I have an idea what might be going wrong here.
How are you actually adding the dropdown object to the screen in question? It sounds like you are adding a generic Dropdown, and configuring it to reference your 'Customer Discount' field. If this Dropdown has already been added to the Opportunity table, you should see it show up under the 'Add Fields' Tab under the Left hand Toolbox on the customize screen and you can just drag and drop it onto the screen! (the field highlighted in the Red box is the dropdown I added to the opportunity table to demonstrate)
If ive misunderstood, let me know and Ill be happy to walk you through it a little more thoroughly.
Thanks Justin.
Regards,
Ben