Drawing polygons on the blank screen

1
Hello all - I am new to psychtoolbox but have some matlab experience (approx. 1-1.5 years). I am trying to create an experiment in which I present visual stimuli as targets. However, I can't even seem to get anything drawn on the screen. I have tried using Screen functions (fillRect, FramePoly) etc, and haven't figured out what I am doing wrong. This is what I have so far:

ExpScreen=0;
window=Screen(ExpScreen,'OpenWindow'); %opens window for experiment to be run within
black=BlackIndex(window); % pixel value for black
Triangle = [100 100; 200 100; 150 400];
Screen('FramePoly', window, black, Triangle);

Thanks in advance -
Sara