Could anybody tell me step by step on how to register those? SE method of handling this thing seem very confusing.

At first I start my PS3, and said that I already have account, and then click on "link" account. but then it turn out I have two account. and then I try to log in my old PC account (CE + Legacy) via PS3, it says that I can not log in via multiple platform, and need to have a proper registration first. I can only log into my 2nd service account which is "PS3 + standard" instead =_=.

Now I haven't register the coupon code with 18 letter about product registration card yet. Where should I register this one and how? will I be able to log in my old legacy account on PS3?