Dear Ghaleon:
Thanks for the compliment about the educational activities.
I have very good news. All summer, I have been updating the Activities and am writing two new ones. One of them, about the Search for Extraterrestrial Intelligence, takes you on a grand tour of the universe, with stops at some beautiful fictional solar systems. It has many new features. The other takes you back in time to the formation of our Moon via collision between Earth and another planet billions of years ago. This add-on is shapping up to be spectacular, with explosions, ring rubble piles that orbit the two-body object, glowing, flowing lava, etc.
I will be updating all the Activities on the Motherlode in about 3 weeks. I am also distributing them via a CD set (2 CD's) which will be offered for sale and will self-install.
I'll be making some formal announcements soon.
The good news is that I have rearranged all the add-ons, and will be using the "new" feature in the latest experimental build of 1.4.0 that enables you to load a different celestia.cfg file. This file tells the program what "extras" to load, and from where. It used to be that the program could only load one such file. All the add-ons you wished to visit had to be in specific spots, and would all load together.
Now, the Activities will load only those add-ons particular to that one Activity. It will save your computer from having to load up them all ... something that locks up even the best computer.
If this all sounds a bit confusing, don't worry. You won't have to do anything except launch an Activity. Everything will be quite automated.
As to your question of placing other add-ons into the Educational-Extras folder, that is very possible, but you have to be careful. Some of those add-ons may already exist in the Activity, under a slightly different name. For example, I have a detailed model of the Hubble telescope in my Activities which loads with all of them (common to all Activities). That same model is available from the Motherlode. If you drag it into the Educational-extras folder and happen to put it in a slightly different place, then Celestia may load two models of the add-on in the same spot at the same time.
So ... to add a new add-on to the educational-extras folder, simply be careful what you are moving in, and try to confirm that it does not already exist in the folder somewhere under a slightly different name.
Also be aware that in 3 - 4 weeks, I will be releasing all new material and will be re-arranging the extras folders. I will request at that time that you discard all existing educational-extras files and reload them again from the Motherlode in their new formats/configuration. I know that will take a lot of time ... but ... it will work out a lot better.
Watch for the updated Activities in a few weeks.
Regards
Frank