It depends if you want the code to be applied to the whole basket of single products. If its single products then use
Promotions/Catalog Price Rules
IF its for the whole Cart then use Promotions/Shopping Cart Price Rules
Ok I guess its for the whole basket giving the customer a 10% discount
So follow these:
Promotion/Shopping Cart Price Rules
Rule Name: 10percentoff
Customer Groups: Select all if you want it to apply to all customer groups or if you just want a specific customer group select that one.
Coupon Code: Insert the code you want to use
Uses Per Coupon: When im using this i set this to 0 as its for the whole basket ( i guess it could also be 1 depends on your purpose)
Uses Per Customer: Again depends on your needs I have 0 but again it could be 1 so they can onlu use it once.
Leave the conditions tag as this is not needed.
Apply: Percent of product Price Discount
Discount amount: 10
Maximum Qty Discount is Applied to: i have this set to 1000 as I want this to apply to the whole basket and I dont think anyone will have more than 1000 products in my basket ( feel free to set this to what you want if you think you need more)
Discount Qty Step (Buy X): 0
Free shipping: No ( again its up to you )
Stop further rules processing: No ( the reason i have no is I also have if the customer spends more than 150.00 they get free shipping and I dont want this canceled out )
Then go to System/Cache Management and clear the cache.
Give that a try and let me know how you get on. Hope it help..