from where i get an license key ?

You don`t need any activation key to get a beta-license. Open the main Antivirus window. Press "About -> License management". Be sure that the point "Beta-testing" is active. Fill the fields "Name" and "E-mail". Then press the button "Get license". That`s all.

i almost do this.... but it expire : Licence expiration date : 08/04/2011

It's right. Duration of license is 3 month.

And after 3 months, what i have to do ? To get an other beta testing licence ?

Yes, probably