ㄴ/은 turns verbs into their past determiner forms, so 앉은 남자 would be "the man who sat" (keep in mind that it doesn't imply that the action is finished, so it can mean he sat, and is still in the sitting position)
This should not be accepted. This sentence means that I am talking with the man who is (in the middle of the act of) sitting down. If you want it to mean "I am talking with the man who is seated," that would be 저는 앉아 있는(or계신) 남자하고(or와) 이야기하고 있어요.
를 is the accusative particle (direct object). 하고 means and/with, so this is more "I'm talking WITH the seated man" (to would be 에게 (indirect object)) but regarless of how you say it people will understand you.