Hi,
If you're trying to load an iolite v3 reference material file, you should use the iolite v3 RM importer tool in the Tools menu (listed as "Import iolite3 RM file" in the Tools menu).
This will ask you to specify the file you want to import, and then it will convert it to iolite v4 Ref Mat format (v4 uses a different format), and save it in your Reference Materials folder.
Please let me know if that doesn't work.
Cheers,
Bence