If your trail Period has elapsed, no Graphical method cab be used for activating your Windows Server. so in such Case you can still use SLMGR.vbs cmd which is located in %WINDIR%\system32 directory. the Server running from 14 days must be activated in windows server 2008 Edition. here is the command.
:- slmgr.vbs -ato
if you wan to activate remotely this is called as Utopia (by default password is password123)
:- slmgr.vbs Utopia Administrator password123 -ato