If you try to add your feed to My Yahoo! but can't, that's because the link on Yahoo's help pages about RSS (where it says "add by RSS URL") is no longer working. You'll just constantly get an "unable to update" etc message, even after you sign in to Yahoo.
But this link does work:
So just click that link and sign in to MyYahoo!
Under the "Add New Sources" heading fill in your feed URL and hit Search.
And in the results page under the "Add New Sources to New Page" heading just make sure your feed is listed there, and ticked, and click "Add".
Technorati Tags: Blogging, Feeds, RSS, Yahoo, MyYahoo