I can't seem to figure this out for the life of me. I'm pretty new to the construction set, but I feel like I'm doing everything right and it's just not working. Here's the issue:

I have a mod that includes custom races and several custom hairs, I want to use the hairs from this mod on a second mod that includes a different custom race.

More specifically, I want to use the hairs in the Xenius Race Compilation with the "Ayleidoon" race included in the Ayleidoon Karan mod.

Here's what I'm doing:
I open the Construction Set, load up the esp files for the two mods, make the Ayleidoon Karan mod the active plugin, then go to Characters -> Races, and select the Ayleidoon race on the left and go to the Body Data tab. Then I go to Characters -> Hair and find the hair that has been added by the Xenius Race mod. I then drag and drop the hair into the "Hair Styles" area for the Ayleidoon race. Once I'm done, I click OK on the Hair window to close it and I click OK on the Race window to close that as well. Then I save the plugin and close the Construction Set.

However, when I open Oblivion and go to the Ayleidoon race, it still has the default hair that came with the mod and no added hair.

What am I doing wrong? I've tried to look it up but I can't find any info on it, I feel like I'm probably making a simple mistake or maybe what I'm trying to do is a lot more complicated or something.

In order for the method you described to work, you would need to make the Xenius Race Compilation into a master file (esm), and then make the Ayleidoon Karan mod dependent on it.

There is more than one way to do what you're asking about, but I recommend you start by reading http://cs.elderscrolls.com/constwiki/index.php/De-Isolation_Tutorial on the CS Wiki. It is very relevant here.

Thanks! It took a few hours since I didn't have Wrye Bash or anything like that and had to work around a bunch of other issues, but I finally got the race I want with the hair I want.

Now to make a character I'll probably spend less time playing on than the time I spent getting it to look the way I want!

(Though the knowledge I got from learning the whole process will certainly be useful in the future, if nothing else )

Hehe, this is the right outlook. Glad you got things working.