These instruments should have been installed when you downloaded the additional content of Mainstage 2.
Launch Mainstage again; and open again the "Download additional Content". Check the status of "JamPack World Music". Click the disclosure triangle, to see the check mark for "Instruments". Enable it and try the download again by pressing "Install". I hope that will work.
Im about half way throught the download
so will try the drag across the folder like last time.
That is only necessary for loops, not for instruments. So it should work right away.
I'll be leaving on vacation tomorrow and may not be able to get online, so hopefully someone else will jump in again. If necessary, write a new question.
You could always try a 3rd party vendor such as my own, http://modeaudio.com. We offer a nice selection of Apple Loops from drums, bass and synths to percussion, guitars and more, all profressionally tagged and formatted.
Our selection is very reasonably priced and you can also download our 350MB+ taster pack for free, in Apple Loops format here - http://modeaudio.com/newsletter. Hope this helps!