Ok I'm not a Metallica fan.... but.... I actually will try to help you....

I'll start off giving the same advice that I give to most people with tone problems..... (because most beginners just don't get it...)

As the others have told you, you need extremely scooped mids to get that tone.... but there's a problem with your gear....

Its your amp - an amp is the driving force behind most tone.....

Let me expand....

For most pros, distortion comes from two things:
The amp and the pedal....
The amp gives the tone, and the pedal just boosts it...
Now almost all of the pros (including metallica) use tube amps..... this gives you the classic distortion that you need....
The amp you have may be a marshall (a great company), but its a solid state amplifier (or their fancy name FDD)...
Solid State and Tube is like the difference between St. Anger and Master of Puppets.... one is crap... while the other can sometimes blow your mind...

But until recently to get this tone you would have to buy a tube amplifier (which are very expensive...)
Now this isn't the case...

Go Digital! A digital processor models amplifiers that are very expensive... while the unit itself relatively inexpensive ....
I reccomend the Digitech RP series....
While using these, you can make sounds that sound like metallica, or any other band for that matter (well maybe not every band.... but thats ok)
But keep in mind that digital processing does not exactly replicate amp models... you won't have the exact, precise tone... but it'll be pretty darn close....

If you don't want to buy one of these processors.... or a tube amp..... then I really can't help you more than ne1 else here did...

(what is it with metallica these days?)