NRFit
Neutron and x-ray reflectivity analysis in IgorPro environment / nrfit
...It will show up in the menu.
Note: if it complains about FONT, select your favorite font and apply it to all experiments. First time users might also want to turn ON the ShowTips from Graph menu. Have IgorPro6.0MAC? Follow the NRfit link and browse the All files, and download the nrfit osx 6.0 version.
Downloads:
Download a proper version (window, mac) from the 'Browse all files' menu.