Ok, to attach a attachment downloaded here, you can make a trigger that attaches it. When your making a trigger go down to special effects then go on Create Effect on Unit, the spot that Says overhead Change that to head. Then set the unit im going to be using a Priestwhich i want it attached to. Then set what to attach in this case im using a Footman Helmet.
Color Coding is:
Green= Part
Blue = Unit
Red = Placement Area
Orange = Trigger Type