Most Helpful Guy
Sex appeal is simply attributes and characteristics of your appearance, your movements, and your personality that are sexually attractive (to men, primarily).
Clothing and makeup should be pretty easy to figure out. If you tend to wear unisex clothes rather than "girl" clothes, and if you wore no makeup and didn't really do anything to your hair, many people would say that it wasn't sexy.
Sexy women move with sexiness and grace. They often walk as models do, with one foot in front of the other, which causes your hips to sway back and forth when you walk. And their gestures are slower and more feminine.
As far as personality, sexy women tend to enjoy being women, and are confident in their feminine aspects.
You're saying you're a Tomboy and more-or-less non-sexual, so it shouldn't be any surprise that people say that you lack sex appeal. That's something you can change if you want to, but it will probably take some work and practice for you.