I have a fix for this

In my installation, the view product.pricelist.website.form had a group set to public. 
Removing this allowed the Promotional Code field to appear in the pricelist.

However the shopping cart still was not applying the discount when I entered the code in Coupon Code.
This was due to a bug, which was fixed in this patch.

https://github.com/odoo/odoo/commit/7a2ba5ca47e896be681c53ef0dc869cbad642a78#diff-cf00d16a9ce1440df9ec66a522633604

I hope this saves you a lot of time. 

I made a complete video of how to activate and create coupon codes from scratch for Odoo 8 ecommerce.
https://youtu.be/Og7l-5CKpBs 

impossible to post with a screen shot! sorry

I my V8 Saas database, the coupon code appears in the pricelist form.

The name of the field is Code

Nom de champ

code

Libellé du champ

Promotional Code

Type

Champ de base

Modèle

Liste de prix

In Modules

website_sale

Type de champ

char

Relation de l'objet

Champ Relation

Options de sélection

Taille

0

Domaine

Champ de sérialisation

On Delete

Requis

Lecture seule

Recherche possible

Recherche impossible

Traduisible