In ADS layout, you use variables by moving vertices. So, if you would like to increase the width of your rectangle, you grab the two corresponding vertices, and move those by the desired amount. If you current width is X, you can specify the variable to have a nominal value of X with a perturbed value of X + the amount that you move it.
First, draw your nominal rectangle. Then go to Momentum\Component\Parameters
Enter a name for your variable, the nominal value, and the perturbed value. Then click "Add".
A new layout will open up, and you grab the corresponding vertices, and move them by the perturbed value. Then click "OK", and you should be set.
Good Luck!