You cannot. The whole point of Passcard and Identity security is that only the person who knows the Master Password can use Passcards and Identities protected by this password.
If you forgot your Master Password (or if you do not remember creating one but RoboForm requests it from time to time) then the only course of action is to remove all Passcards and Identities protected by Master Password and then remove Master Password itself.
In "Options -> Security" click "Set Master Password, Mass Protect/Unprotect" button, click "I forgot it!" button and follow the instructions.