*Geeezus...*
Alright, you should have
1-a .Mdx file
2-a .blp file
Now extract them both and import them on Import Manager. Double-click on the .blp file and apply the path displayed on the page where you found the attachment model on after checking the "Use custom path" box. Now, go in the Trigger Editor and use the "Special Effect-Attach *your attachment* to the *weapon, or left hand, or right hand* of unit".
Try learning on your own and read the tutorials before asking that.