Installation is simple, just unzip into NWN root or give the content of the archive to your override.
Well this icon set was originally created for my own prestige classess derivated from PRC. However I don't know if I ever finish that, as with every new class I added, I found out I'm stepping into exact issues PRC got and its more and more complex...
So you don't see few icons ingame because:
Some prestiges dont gain "Epic Class" feat on lvl 11, thats like Acolyte of the Skin or some feats aren't in PRC because I made different based on 3.5 rules which in some cases didn't matched with PRC. But all icons you see on image are included anyway.
If would be interest, I think that original author, Gerder, will be able to make some new icons.
Btw check out for icon prefix:
ir - class icon
is - spell icon
aots - Acolyte of the Skin
arch - Archmage
bfz - Black Flame Zealot
dod - Disciple of Dispater
dom - Disciple of Mephistopheles
pa - Peerless Archer
rav - Red Avenger
sb - Shining Blade of Heironeous
tn - True Necromanacer