Usually if you mouse-over the My eBay link at the top, the coupon will be listed there at the end (I haven't had one for a while, but apparently it doesn't show if you mouse-over while logged into the forums, so make sure you're on a proper eBay page to check if it's there). 

Sometimes they're global promotions (i.e. available to everyone) that they advertise when you sign in to eBay (eg the other week I signed in and before I was taken to My eBay, there was an ad for 20% off at Target). If it was one of those kind of deals, you can usually find it being promoted in other areas of eBay, either on the home page, or on the 'eBay Deals' page.