Type the command add sipb to attach the sipb locker and add it to your search path. If that doesn't work, then there is something non-standard about your account configuration; try using these commands instead:
attach sipb
set path = (/mit/sipb/bin $path)
After using one of these, typing rn will bring up the text
interface to the Usenet, or typing xrn will bring up the X
interface.