You should be able to do that by using the object designer and setting the string sub-type of the process/process assignment/Description attribute to be html. If you also change the assignment window so that you show the editor, it should make life a bit easier in future too.
You will change all assignments for all process objects by doing this, so just be aware of that.
Thanks for the response.
I'm not able to edit the settings using object designer. Its all disabled. How should I proceed?
Pop up a screen shot and let's have a look. As long as you are looking at that process object, you should be able to turn HTML on and off any time