adding curve line in a sq.

for my illustrator class we have to make cards. So for the diamond when i tried to make it curve in it makes the lines come to a point instead of a curve.
I make a square then i add an anchor point and use the convert anchor point tool and drag... and it doesn't work. I'm able to do one side but the others
What am i doing wrong?

anyone

i fig. out what i was doing wrong... thanks for your help