Edit in JSFiddle

window.onload = function(){
    //canvasを取得
    var canvas = document.getElementById("myCanvas");

    //空のproject,viewをcanvasに作成
    paper.setup(canvas);

    //線を描画
    var path = new paper.Path();
    path.strokeColor = "#000";

    var start = new paper.Point(100,100);
    path.moveTo(start);
    
    //addを使う
    path.lineTo(start.add([200,-50]));
    
    //viewを描画
    paper.view.draw();
    
}

<canvas id="myCanvas" resize></canvas>