The access method varies depending on what Android version you are running, but in general, go into the address book (i.e., Contacts), press Menu, and then it will either be nested in the "Settings" menu or there will be a direct link to "Contacts to display." Next you will see a list of all your contact sources - Select "Customized list" if it is not already. This will kick you back one level in the menu. Now press the gear icon to the right of "Customized list" and press the Yahoo account to expand it. There will likely be one or more boxes unchecked, and you can take it from there. The other possibility is that you have it set to only display contacts with phone numbers, and by coincidence your Yahoo contacts don't have any. If you think that might be the case, again from the address book, Menu > Settings, and uncheck "Only contacts with phones."