Actually I'm not so sure what is the difference between orthotropic and orthorhombic. I guess you should take orthorhombic specimen symmetry and add the option
antipodal to your plot command, i.e.,
plotpdf(odf,Miller(1,1,1),'antipodal')
To see the symmetry, you probably want to do
plotpdf(odf,Miller(1,1,1),'antipodal','complete')
Hope this helps,
Ralf.