Question...

How would I add a streamer thing to a Demon Hunter's blades but show it only when he attacks?

I'm assuming you've already added the ribbon itself to the model and are only asking how to make it ONLY activate when the unit attacks. The solution is that you need a "visibility" parameter in the ribbon. Here's an example from one of my units:

RibbonEmitter "BlizRibbon01" {
ObjectId 65,
Parent 44,
Visibility 3 {
DontInterp,
167: 1,
1833: 1,
4500: 1,
7000: 1,
12500: 1,
13667: 1,
17667: 1,
18833: 1,
20000: 1,
21333: 0,
25000: 0,
92200: 1,
159967:0,
}
static HeightAbove 3,
static HeightBelow 3,
static Alpha 0.4,
static Color { 1, 1, 1 },
static TextureSlot 0,
EmissionRate 75,
LifeSpan .7,
Rows 1,
Columns 1,
MaterialID 3,

}

In the above, each "1" corresponds to a keyframe when the ribbon is visible. Mine is slightly different because I wanted it to always be on (except when dying) but for what you want, simply set all animations to 0 except the attack anaimations (there may be more than one).

actually, I need to know how to get the ribbon attatched, too :\

Ribbons work almost exactly like emitters do. If you look above at the one one I pasted, you'll notice it has most of the same type of syntax. Hieghtabove and heightbelow control the thickness of the emitter. In general, the only other parameters besides those you should play with, probably, are alpha (how transparent the ribbon is) and lifespan (how long it lasts).

One thing to note is that ribbons use materials, not textures. You probably should open the paladin or blademaster for reference - they should both have a material that's using a texture that I think has the name "ribbon" in it. Of course, ANY texture could be used, some probably with pretty cool results.

Edit: I'm pretty sure they both use "Textures\RibbonBlur1.blp"

Anyway, the only other thing you need to know is that ribbons must come after particle emitters but before collision shapes and event objects (RIGHT after emitters, in other words) in the mdl.

Once you have the ribbon set up, give it the proper objectID, you'll want to attach it to something so that it swings with the sword. You might need to experiment a bit, but I'd suggest making it's "parent" be the objectID of the weapon attachment point (which is located right above the list of pivot points) and make the *ribbon's* pivot point have the same coordinates as the parent (in this case, the weapon attachment point).

Hope that clears up some stuff about ribbons. Let me know if you have any other questions.