Hey Toga,
I'm not sure how you made out, but I created a reusable version of the code that works with multi-select lookups ,
you just need to call a method
setupAddNew(
"Material Type","TypesOfMaterial","/Lists/TypesOfMaterial/NewForm.aspx","Title","ows_Title" ,true
);
That last parameter of true, makes it work with multi-selectes.
http://code.msdn.microsoft.com/Javascript-to-add-an-Add-60b6299b
hth,
Russell