I was having the same problem. On that page of questions from a variety of categories that you see, click on any category. It will bring up a tab where you can click on "all categories".
Be patient, Yahoo is redesigning categories. There will now be just 2 categories,
I. Intelligent questions'and responses
2. Stupid questions and responses
.
You can't right now. There is no way to do it but you can get to the categories but it a little harder.
If you want to find a category on this new terrible format just go to the top of the page and Click on Answers Home. Then a new page will open and you have the All Categories list. Now you can just go to the category you want. Easy but it shouldn't be this way.