Simulations created with Physlet library need javascript to generate actions.
You can not just add applet tag(It would not work, unless you did not need to add any objects or actions).
Please check out the html source from generated physlet simulations and you will find those javascript code.
And when you submit message, the forum  will change line break(RETURN:which is not visible) into html line break code.
So (RETURN) will add some code in the applet tag or javascript. You will need to remove it.
And the physlet classes are located at "/classes/" at this web site.

Click in the simulation regin to draw field line which will pass the click point.