Does "We are yet to receive your $300 check for the conference fee in the mail" means they receive my payment or not?
"We are yet to receive our check" is a polite way to say "we have not yet received our check." It is a somewhat old-fashioned form, but its meaning is that we have not yet received it. Quite frankly, it is normally expressed "have yet," rather than "are yet." If one states that he has yet to hear from another, then it means he has not heard from that other person yet. Whenever the word "yet" appears, it is safe to assume that there is a negative connotation somewhere. Examples: I have yet to grade your papers; I have yet to receive my paycheck this week. A bit old-fashioned, but the meaning is always "are not yet" or "have not yet." It simply turns a negative statement into a positive one, without changing the meaning.
I can see your confusion. The key word here is "yet." The meaning of "yet" is rooted in time. We can say that "yet" means: up until the present or a specified or implied time.
So, when someone says that "they are yet or have yet...," the word, "yet" functions as a negative adverb. Let me give you a few examples.
I have yet to go to Disney World = I have not gone to Disney World so far = I have not gone to Disney World.
We have not yet purchased the supplies = I have not purchased the supplies so far = I have not purchased the supplies.
From this framework, your sentence means that the persons in view have not received the 300 dollar check for the conference in the mail.