How do i select a commander when building a deck?

[Rayburn8]

Need to know.

[Fizanko]

By default you're in the Main part of the editor, so click on the Change Section button in the commander deck editor :


Until you reach the Commander part of it :


And you will be able to add your commander there.
Then you can go back to the Main part by clicking on the same Change Section button (that can be used to go to the Sideboard part too)