In Othello, why has Lodovico come to Cyprus, and how does he become a part of Iago's scheming?

I absolutely agree with kwoo1312, but I would add that Lodovico is also critical to the plot because he is a representative of the Duke.  Earlier in the play, the Duke of Venice sided with Othello, even against a nobleman (Desdemona's father).  As long as the Duke is firmly in Othello's corner, his position is secure.  However, if the Duke turns on Othello, then he is far more vulnerable.  If Iago wants to totally destroy Othello, he has to undermine that respect.  If Othello had not killed Desdemona and they had all gone home to Venice, Lodovico would have carried word to the Duke that Othello had lost all honor and become abusive.  That would have been very damaging to Othello.  

So, in the play itself, he plays very little role in the schemes.  He is the one who finds Roderigo injured and accepts Iago's explanations, but he doesn't advance Iago's plans much at all.  However, he could potentially do Othello a lot of damage back home.