Dear @luillo
Sorry for the late reply but i was on vacation these past weeks.
In the meantime, i tried your suggestion and I even formatted my mac and reinstalled Macports and FLUKA from scratch but i got the same exact symptoms.
Still the same crash and pkg-config is unable to locate tcl before explicitly exporting the libraries path.
Moreover, I saw that other people have found the same issue (FLAIR 3.4-5.4 crashes on Mac ) so I suspect that something broke in the tk package distributed by Macports.
Cheers,
Matteo