It depends on what browser you're using. Mozilla Firefox will need margin: auto; inside of the CSS (As you have now), but IE is the short-bus-rider that needs special attention...
For internet explorer, you have to add in text-align: center; to your css, and I noticed that you didn't have margin: auto; in the parent element that you wanted to center. Adding the simple attribute align="center" into the tag itself works in most cases.
Hope this helps!