Can Mephesto target your opponents graveyards? I think he can but some people disagree with me and I just want to be sure.

yes he can. Read the card.
I don't know what will happen if 2 players have mephisto (5) and both try to use his effect tho....

player A : I will reveal
player B : In response I will reveal
Player A : Wait I'll reveal again
player B : Oh yeah, what if I reveal again ?
player A : cool, I'll just reveal one more time
player B : you son of a @#$% I'll reveal again
player A : ....... (standing up) I'll reveal one more time
player B : ........ ARGHHH !!!! (jump over the table and beat the hell out of player A)

hmm........

man this set is full of cards that allow you to draw the game lol.

that's been errata'd. Essentially, (cause the explanation makes my head hurt) whoever is the second person to use a Mephisto wins.

I looked for an errata before posting and I could not find one for this card. I did find one for his 8 drop but not him. Please point me in the right direction.

You can find an extremely full discussion of the intricacies of Mephisto, Soul Stealer here, including comments from Alex Charsky towards the end. There has been no errata, but Steven_Q is essentially right on how two players' Mephisto uses would resolve, assuming that this second player is prepeared for it, and the next update to the Comp. Rules may make this more clear.