So, for you. Go to your profile page. Click on Edit. In the text at the top (below the tabs but above your name) there will be edit DNA TESTS HERE. Click on that link. There you can enter the information about your mtDNA test.
For your brother, you will have to add your parents first. Then add his profile. If you mark him as deceased, you can then do the same as you did for your profile and enter his test information.