They are in the VFX list on the creature's properties. It's just like picking a normal spell vfx, ghost vfx, stoneskin vfx, etc. They can also be applied via script. You will find them in the sp_skin_1 to sp_skin_222 range of the visual effects dropdown when editing creatures.
This thread has about 60 of the skins displayed: http://forum.bioware.com/topic/369765-kaedrins-supervfx-pack/?hl=kaedrin#entry11424807 (link is external)
The images below show the process and three of the skins being demonstrated.
The first shows where you pick the VFX. VFX2 is the original mindflayer. VFX3-5 are just 3 of the skins/vfx applied to the same mindflayer to give it a different look/purpose.
Using these you can make plantlike creatures that are using a different model, partially shadow or elemental creatures, various types of stone creatures, etc etc. All of the skins I did start with sp_skin_ so you can just paste that into the VFX_ line and you jump straight to my skins. I think the sheer variety and amazing look these can add would really punch up the game for a lot of players without even getting new models (just by re-using the models we do have).
I can also generate new skins in about a minute a piece once I get the tool set back up again. I just need a high resolution image (the sky, metal textures, web textures, leaves, ANYTHING) and I can turn it into a skin. The original format (jpeg, TGA, BMP, etc) doesn't matter as I convert them to TGA and then into DDS files. There are 3 files per skin (1 tga, 1 dds, and 1 sef).
The bigger the model, though, generally the worse the skins look *unless* the models were neatly broken into subpieces. It's hard to explain but as you experiment you will see. What looks great on one model can look like crap on another.
I hope you enjoy it and if folks want more I enjoy creating these as a nice creative diversion. You can download the VFX pack from either http://neverwintervault.org/project/nwn2/other/visual-effect/kaedrins-supervfx-pack or http://www.nexusmods.com/neverwinter2/mods/1007/?