From within the code in your form called MainMenu, you should set a variable elsewhere, using the Me keyword to refer to the instance of the class from which the code is running.
In the case of a main form, it's often best to declare a static ("shared" in vb) public variable in the form and set that in the form's constructor, since there will only be one instance. Other forms can then use this shared reference to get the instance of the main form from anywhere.