If you generated a Type 1 font for Mac, there will be two files: The font suitcase (“FFIL” on the icon) and an outline font file (“LWFN” on the icon). The error indicates that only the suitcase file was installed.

Normally I double click the FFIL file and the font is installed in Fontbook. I don’t know how FontExplorer works, but my guess is that it works the same. How else should you install both files?
If you check the destination folder for the fonts, both files are in there.
Is there another problem that might initiate this error?

The sure fire way is to go to your home folder, then go to Library > Fonts. Drag both files to the Fonts folder. This is essentially what Font Book does when you tell it to install a FFIL file you double-clicked on. I knows about the companion LWFN file automatically. FontExplorer’s default behavior is to make a copy of the files in its private database, but you have to drag both files to it, not just the FFIL.

Generally, when I’m testing fonts, I don’t use a font management tool. I copy files manually to either my user Fonts folder or the Adobe Fonts folder ([main hard drive]/Library/Application Support/Adobe/Fonts/).

Thnx Mark, I’ll send this solution to my friend and see if it works.

Ok, quick reply. Both font files are in the right place. FontExplorer seems to have an own ordner per font, so there must be another problem at work.
Can the LWFN file be corrupt? Can it be that, when generating the font, the outlines are not put out?

Strange...

I’ve never heard of an LWFN file being corrupt, but I suppose it’s possible. In the pre-OS X days it was common for FFIL (suitcase) files to become corrupt because they were always open for read/write access when they were active.