FEM-meshing and FEM head model

What do you mean by "it is not properly installed"?
What do you have in the folder /Users/yvonnebuschermohle/.brainstorm/iso2mesh/mac64 ?

If you remove iso2mesh from the Matlab path, and execute the following command, what do you obtain?
process_generate_fem('InstallIso2mesh', 1)

@MartinC Could you try to execute the same command on your Mac?