you need to link your wow account to battle.net .then use the email you registered as your account name,and your password as usual.was implemented a week or so ago as mandatory, please refrain from sharing those details with anyone regardless of who people say they are.
so head to battle.net , register,log in,and link your wow trial , then all should be well , unless your free 10 days is up , in which caseyou need to pay for gametime and join us bleachers ;] , or have a real life and miss out
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hope this helps