The original image is squeak-dev 95-2 updated a few times, so I did not install OB but simply updated it along with other packages. Have to admit to a bit of confusion since the Package Universe Browser shows the following installed (and the fact that until this doc was posted I ignored OB altogether):
OmniBrowser 0.337 OmniBrowser-Full 0.7 OmniBrowser-Morphic 0.5 OmniBrowser-Standard 0.183 ShoutOmniBrowser tween.5
Things I may actually have installed include: null, DuoSystemBrowser, VPVideoMorph, MultiColumn Lists, OSProcess, RIO, GraphViz, FSM, Connectors... I think that's it :-0
Happy to help you track down what has happened but since my Squeak/ST experience is patchy a bit of hand-holding will be needed.