I don't know of any guides, but I'll take a look. Really, I just taught myself how to use it by taking a picture of something and trying to make a 3d model out of it.
You can try the demo here:
http://www.thegamecreators.com/?f=ac3d
AC3D is more expensive than some other modeling programs, but it's more than worth the price; making models with it is a breeze. The only downside is that it doesn't have any animation features.
On a side note though, there's this great new feature it has where you can take a grayscale image, and it will convert it into a 3d object, which would be great for making terrain.
Edit: Here's that feature in action with a random height map I pulled off the internet. I doubt you'd be able to make this into a segment directly 'cos it's too big, but you could probably import it into something like 3d world studio or something like that and then bring it into X10...
H.K.