How do I get
inside? 
 You'll have to resort to trial and error.  There is no way to boot the machine
 into a single user shell without an external boot device (and this will be
 hard to find, as detailed in earlier messages). 
 
I vaguely remember that the RS6000 machines could be booted into a
maintenance mode through use of the keyswitch on the front.  It's been a
long while though, and I'm not sure whether all the models had the
keyswitch (the 500 & 900 series certainly did).