The question was about MAKING BUTTONS.
You could make two small planes, objects 200 and 201 say, place them on screen, and use PICK OBJECT to check whether they had been "picked", i.e. clicked on.
If object 200 was picked reduce the effect constant by a small amount, and if 201 was picked increase the effect constant. If neither - well don't change the effect constant.
However, you are using a full-screen effect so PICK OBJECT might just pick the screen quad. I haven't got DBP handy here so I can't check for you. Could you be more specific about the kind of full-screen effect you are trying to use? Some simple code would help.
Edit Just noticed the original posting date.
Oh well ...