I'm going to give you some relatively brief answers, as I don't know much about modelling. Brief, maybe, but answers nevertheless.
For the monkey, I would assume you're correct, it probably doesn't have attach points on it's hands. A good way to check is to download reyk's new editor and check the attach points with the animation viewer.
For editing the attach points, that requires editing the models. Like I said, I don't know how to do this. Try contacting coolsk8r32again or argalius, to name a couple. They may be able to help you if you can figure out the fundamentals.
For the tutorials, I know that there are for the GMAX program. They're about 10MB, I believe, and they're free, as is the program itself. Look in the Modding Resources section of this site for a link.
For the "path of animation", as you call it, you must edit the models as well. Namely, you'd have to move the various vertices and what not for each animation frame into the position you want... tedious work, as far as I know.
There are a couple ways. If the unit has an attach point existing on it's back, simply define an animation for the unit you want to ride in the riding unit's anim file, then connect that defined unit to the attach point. If you can figure out how to do it, add whatever attach point you need to make it look right. Another way is to simply try to modify an existing riding unit's model, like a Hippikon or whatnot.
Well, that's all I can offer.
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Aw, hell, I dunno, she had some tig-ole-bitties, d'int she?
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