Did you enter your code in the window, on the plateia presale page, before clicking on "validate"?
I don't know what to tell you, and i have to go to work right now. Good luck.
Here is the link [
www.plateia.iol.pt]
If it doesn't work maybe you should check your internet options and be sure that the anti pop up is off. Sometimes it's a source of troubles with ticketing services.
Good luck.