Click on the Windows button in the corner of your screen.
You should see 'Computer'. Click on that.
Click on 'Local Disk C:'.
Find 'Program Files', open that.
Scroll down the list until you see 'Steam'. Open it.
Find 'Steamapps'. Open it.
'Morrowind' should be here. Open it.
Open 'Data files'. Place the Accurate Attack.ESP in there.
You should be at the launcher menu. Click on the button 'Data Files'.
A list of boxes should pop up. Enable Accurate Attack's box.
Hit the play button. Enjoy hitting 100% of the time.
Follow these steps and everything should be fine. Your C: drive may be different. If you use a different drive for games, I can help you there too. But this should suffice assuming you haven't played around with Steam's settings.