What was Jesus trying to convey in the parables of the mustard seed and yeast? What message did Jesus Christ tried to give by comparing the Kingdom Of God(Isn't that Heaven?) with the mustard seed and yeast?
Jesus was saying that the Kingdom of God has a huge effect on whatever it interacts with.  That is, the effect is disproportionately large in comparison to what we mortals perceive the Kingdom of God to be.

The Kingdom of God refers to more than just Heaven.  The Kingdom of God also includes his people, many of whom are still here on Earth.