I had the same problem yesterday. After checking the PES forums, I discovered that what schallwert advises is correct.
Sign in on FilePlanet (www.fileplanet.com), then download as usual.
You have to go to www.fileplanet.com and log in there (you should already have an account if you've downloaded from PES before, it's not the same account as the one you have with PES itself) or register a new account (free is still available).
Then go back to PES and try to download again, this time you won't go to the IGN Prime registration page (which does NOT make any mention of free registration or logging into existing accounts).