\Cc0c0c0To activate ZBrush, press \Cffffff'NEXT'\Cc0c0c0 and choose the file called 'ULicense.zul'. It is located in the folder occupied by your registered copy of ZBrush 1.55b.

If you have not yet registered ZBrush 1.55b, please exit and follow the instructions in the 'Serial_Number.txt' file (accompanying ZBrush 1.55b) to register it now.