how do i put a fraction on the calculator

It doesn't need a special buttom. Just write like this way...

7/2+3/4

If you write this...

$${\frac{{\mathtt{7}}}{{\mathtt{2}}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{3}}}{{\mathtt{4}}}} = {\frac{{\mathtt{17}}}{{\mathtt{4}}}} = {\mathtt{4.25}}$$

You're welcome