I understand how to kasher the microwave oven, but what about the glass dish inside the microwave????

Shalom. You very simply clean it well, then you then do not use the dish for 24 hours prior to kashering. Afterwards you pour boiling water over it and it's ready for use for Pessach. As an additional stringency some people do not place food items directly on the glass but place paper or other material over the dish once it is kashered. Chag Kasher Ve'samea'ch.