To get your IGNOU ID card you need to visit your IGNOU regional centre. You can collect it from there. The ID card should have a stamp of IGNOU on it which makes it valid. If you will download your IGNOU ID card online, it does not have the stamp making it invalid. This will be a major problem in your exams.

It is highly possible that IGNOU does not mail your card. Most of the time you will have to visit the regional centre personally to get your IGNOU ID card.