The steps to add and remove forums to/from "My Forums" has recently (July 2013) been improved, as described in this Wiki article:
http://social.technet.microsoft.com/wiki/contents/articles/18476.msdntechnet-forums-how-to-addremove-forums-from-my-forums-list.aspx.
This isn't a button, but you can hoover your mouse over a forum name and select "Add to my forums", or if the forum is already in your list, select "Remove from my forums".
Richard Mueller - MVP Directory Services
