Currently, ISTool only supports installing TrueType Fonts via the menu entry <Project>-><Install Fonts>
We need to install Open Type fonts (.otf), too. It works if we rename the files to .ttf but then its difficult to make sure the .otf originals were not installed before.
The major thing that would need to be done is to allow .otf files to show up - the internal logic seems to work for them, too (looking up the font name for the filename of the font)
It would be great if you could find the time to do that change.