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Open firmware is a command line environment. Like most command line environment, each command is followed by hitting the return key to activate it and is greeted by a prompt on the screen. So once you are greeted with an identical prompt to what you were when you typed:
reset-all, set defaults, or reset-nvram, type mac-boot followed by a return key.
The
0 >
is an example prompt you may be greeted with, where you would type the command you want to type.