Assuming you're doing any kind of serious riding (say, 100 miles per week) your caloric intake is woefully inadequate. Your basal caloric rate assuming no exercise is probably around 2000 calories per day! Other than short-term "newbie gains" you simply cannot gain muscle if you are not gaining weight. And you cannot gain weight unless you operate at a caloric surplus.

Also, you don't need more muscle mass in your legs to improve your hill riding abilities. The best climbing cyclists in the world are teeny, thin guys. However, if you want to add muscle you need to do three things immediately:

(1) Make sure you are eating 500 calories more per day than you actually burn. First, find out how much you burn when doing no physical activity. It's probably around 2000 calories per day. You probably burn about 40 calories per mile on the bike, give or take. Don't forget to add any other physical stuff you do.

(2) Eat *at least* 100 grams of protein per day, preferrably 125 or more. You also need at least 50 grams of healthy fat. Lean meat, whey shakes, cottage cheese, olive oil, nuts, etc. Eat 6 small meals rather than 3. If after meeting your protein and fat needs for the day you still find yourself at a caloric deficit then drink soda, eat pizza and ice cream, etc. Seriously.

(3) You need to do heavy weightlifting to gain mass: squats, deadlifts, straight-leg deadlifts, calf-raises. Shoot for 2-3 sets of about 8 reps to failure.

Again, these things will help you build muscle but they won't make you go up hills faster. To go up hills faster you need to do interval training on the bike. Sounds like you're not eating enough and not riding enough.

--Steve