How to become a registered apple prototype tester

There is no means of "registering" as far as I know. You can pay to become a registered developer and have access to pre-releases of software that way. People also get invited by Apple to be seed members for pre-released software. Many developers also participate in Apple events like the annual developers conference, and so become known to Apple that way. Invitations are sometimes based on recommendations by long standing members of the developers community, by a history of participation on these forums, by completing professional training courses offered by Apple (they have been know to subsequently invite such people to seed projects). There may be other ways Apple becomes aware of your potential usefullness in pre-release testing (eg. your job or professional standing, such as being a Sys. Admin of Apple hardware systems in a Corporation, University or School).

Basically, it is a system of don't call us, we'll call you if we are aware of you and think you have something to contribute.

Then the only way is to apply for a job to work for Apple - they don't do any hardware testing with non-employees and even then, my understanding that only senior employees with a long standing with the company are likely to ever get their hands on actual complete prototype devices, so be prepared to work your way up to that.