A. The official from X association
The person that I am talking about is coming from that association. I probably expect that person to act, while they are here, on behalf of that organisation.
B. The official for X association
That is one of our own people, not a member of, or acting for that association. It it the person, that for us, keeps in contact and deals with that association.
C. The official with X association
More or less the same as A), but in this case it is certainly true that the person is here in their role as member or representative of the association. You might hear this phrase on a news-item, if someone is called upon as an expert: "And to answer some questions about X, here is ms. Y, who is with the X association."
D. The official at X association
The person is working for, or physically at the premises of X association, but I am not necessarily talking to or otherwise communication with them.