I installed Omeka on my iBook G4, in order to test it. I installed ImageMagick thanks to Fink (http://fink.sf.net/). convert is in /sw/bin/convert. I can use it directly from Terminal.app without any problems. But Omeka is stil telling me that ImageMagick is not properly configured. In the settings, I wrote "/sw/bin/convert" without the quotemarks.
iBook G4, Mac OS X.5.4, Fink 0.9 updated, MAMP (Apache, MySQL 5, PHP5), ImageMagick 6.2.8