Type /cast Feed Pet and click on a piece of food in your bags. If that works, then you've learnt the skill properly, you should be able to find it in either the General or Beast Mastery tabs.
If not, you didn't complete the quest that comes after the one which gives you the actual taming skill. Go back to the trainer you finished that quest at and take his next quest.
Assuming you have trained it, and for whatever reason cannot see the icon, perhaps it's a UI problem? Every so often, I cannot feed my pet. Dragging food over, using the icon etc, has no affect. But simply relogging fixes it