How do I use the apple of Eden? The game tells me to hold square to use it. When I do it releases some kind of yellow shockwave that doesn't seem do to anything. It seems to be killing people at random. I currently need to kill 5 guards and I cannot get the last one to die. What the fuck? Couldn't they have bothered to fucking explain how it worked? Christ. Please help.

Hold the button down until all of your health dissipates, and everyone around you will drop dead. It's not rocket science (but it is ancient magic).

I didn't even know you Apple was an item you could use in game. 
 
Great.

The apple will also make enemies start attacking each other.  
 
This was probably the worst part of of the game. It was meant to be empowering but I just felt like I had handcuffs on.

@TooWalrus: Ok I figured it out. At the start of the charge the apple emits a yellow shockwave that, when released, will make the guards attack each other. However, if you hold down the button a little longer, a white shockwave is emitted that kills guards. :sigh: 
 
The game could have explained it better.

"The game could have explained it better. "

I think it says "use apple" with the key assigned to it on screen, doesn't it?  
 
Ohw right; i was just gathering all the guards arround me then use the apple and keep holding the button until you have no health left (i think it stops) Ezio drops to the ground most of the guards get killed and when you get up; run around until your health replenishes. Then just do it again; pretty straight forward. 
 
Is the title a spoiler?