Does "We are yet to receive your $300 check for the conference fee in the mail" means they receive my payment or not?

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.