I think you can fill out your I-130 form and drop it off at the Islamabad embassy, I read it on the Pakistani/US embassy website. But if not, the Chicago address is probs correct.
And no, you don’t need a landlord’s letter. When you provide evidence of the person supporting you in the US (I’m assuming it’s your fiance’s family), then they just have to be able to show that they’re financially stable and rich enough to support one more person. This is usually done through tax papers, propery documents, bank statements etc.