Under Rule Information, make sure you have the rule status active, the right websites and the right customer groups. Also, double check if you are using coupon codes.
Under Conditions, you don't need to specify anything, since you will always have 15% discount for this product. Just remove that condition that you have.
Under Actions, you have everything right. Just double check if the SKU is right for the product that you want.
After that, go to
System > Cache Management and clear all your cache.
IMPORTANT: Shopping Cart Price Rules just show the discount in the Cart Totals table, NOT on the product.
In my example, I have a product that costs $10.00 and have 10% discount. You see that the product still shows $10, but the Cart Totals table shows the 10% discount:
If you want the discounted price to show in the product, use
Promotion > Catalog Price Rules instead of
Shopping Cart Price Rules.