Dear Karim,
In order to make zsa.objects work correctly in Max 6, the OMax folder should be installed as a package (see Cycling 74’s webpage about this feature).
That means that the OMax folder should be place (on a Mac) either in /Applications/Max 6.1/packages/ or in the Max/packages folder located in your user’s Documents. This way, Max 6 will correctly and automatically detect all the elements of the package including the externals and zsa.descriptors.mxo extension.
That should solve your problem,
Best
Benjamin