Setting global_registers "on" in PHP is not a good idea, and can very easily introduce significant security vulnerabilities to your server. Please don't do it!
This is especially true if you are using others' code, and if you do not really understand the security implications of the setting.
Ideally, the script(s) you are using should be re-written to function without the use of register globals (they will not even be available in PHP6).
That said, setting your domain to run PHP4 on DH will give you a PHP4 environment with register_globals set "on".
Or you can compile your own PHP, or use a copy of DH's php with your own custom php.ini, to accomplish this. Both methods are described in the DH wiki, though they are considered "advanced" and will not be supported by DH tech support should you "bork" it.