You can purchase the Meinrad Font here. There are instructions on how to use them included. I think that the Meinrad fonts are rather laborious to use, but they still seem to be the best solution available at the time. I think that Gregorio will eventually be the best method, but they need to develop a good GUI first (using it currently requires working from the command line!)

I don't understand why no one has come out with a font of all the note shapes that would be compatible with Finale. The basic program has been out for about 20 years. There are many fonts available for it, including guitar chord fret symbols. All of the music engraving functions are there, include selecting how many lines you use on the staff. Its lyric tool is very good. I guess I'll just wait till it happens. It's out of my realm of influence.

I've used both Caeciliae and Meinrad, and I think Caeciliae is much more logical. The codes for creating neums include numbers for the position of the note on the staff and letters that correspond to the kind of neum. So after you've set a few notes you can remember what you're doing without consulting the instructions. Meinrad's codes are completely random or seem so to me.

Caeciliae was not fully functional when I attempted to use it with Word, but with Quark or InDesign it's perfect.

Consider checking this page and also this page (both from the same site) with several free fonts for typesetting scores of gregorian chant. Both Festa Dies (and its upgrade Joseph Potier) and Cecilia are included. There are tutorials too. I do not claim that using these fonts is easy, but it works. (The site is in Spanish, I'm afraid. The author, Juan Andrés Alzate, is Colombian.)