plotParametric doesn't have any stoptime argument. This should work,

plotParametric(x,y);

Adeel.

Thanks a lot, BUT the result is not as it is expected

model VanderPol "Van der Pol oscilator model"

  Real x(start = 1) "Descriptive string for x";

  // x starts at 1

  Real y(start = 1) "Descriptive string for y";

  // y starts at 1

  parameter Real lambda = 0.3;

equation

  der(x) = y;

  // this is the first equation

  der(y) = -x + lambda * (1 - x * x) * y;

  /*The 2nd differencial equation*/

end VanderPol;

And after that book says that I should type  the following:

simulate (VanderPol, stoptime=25)

plotParametric(x,y, stoptime=25)

and get some kind of spiraloid. Insead of that I get only a simple plot. Iа I change the time of simulation, nothing happens.

Because you haven't simulated the model long enough. Its,

simulate (VanderPol, stopTime=25)

not

simulate (VanderPol, stoptime=25)

Notice the capital T in stopTime.
After simulate command send plotParametric,

plotParametric(x,y)

Adeel.

Sample program+output:
http://dev.openmodelica.org/~marsj/forums/vanderpol.png

Thanks a lot!!! 
Everything works as at the picture http://dev.openmodelica.org/~marsj/forums/vanderpol.png

Only one question remains - How can I link a picture to my post?