Firstly you need to get to the first negotiation period in the first season before they become available to buy. You obviously need to have unlocked them in the PES shop.
Also, in PES6 you can choose wether or not you want classic players to appear in the ML. So if you have that option turned off then you won't see them. Not sure if you can change that option once ML has started (If you can there should be an option for it in the "General Settings" part of the ML bar (where you can set difficulty etc).