There isnt a setting to show special instructions on the product details page are you referring to the custom listbox or textbox?

I just tested both of the custom listboxes and they both display fine.

Product Detail Custom Listbox

In order to display the custom listbox on the product detail page do the following:

1. In the cart admin settings under the confirmation page settings
   1. Check the box titled "Show Listbox"
   2. Type in the listbox label
   3. Type in the listbox text
2. In the product setup check the box titled "Show custom fields on product detail page" - this allows you to turn on/off the listbox by product. This is probably the step you missed.

Confirmation Page Listbox

In order to display the custom listbox on the confirmation page do the following:

1. In the cart admin settings under the confirmation page settings
   
   1. Check the box titled "Show Listbox"
   2. Type in the listbox label
   3. Type in the listbox text

The custom listbox on the confirmation page is displayed on the confirmation page before payment. Maybe you were looking for it on the confirmation status page after payment?