how to make a A2 in inDesign? (cant see mm)

And for any open document you can right click an individual ruler, or the intersection box where they meet, and select the units you want to use from the list. I find it sometimes is useful to change units temporarily for some spacing tasks, but it aslo is a fast way to "correct" docs that use the wrong system.

You can just type the measurement followed by MM to indicate milimeters

With no documents open go to Edit>Preferences>Units and Increments and set the measurement system you want to use.