how to specify the position of fields in a form

Hi, 

I would like to create a form containing 4 buttons and specify the position of each button in the form. I wrote the following code:

 Form1= hiCreateAppForm(

?name 'Form1

?fields list(

        ConfigBouton

        1000:0

       500:500

        CompareBouton

        1600:0

        500:500

        AnalogBouton

        1000:1000

        500:500

        RFBouton

        1600:1000

        500:500

)

)

Each time I execute this code, I have the following error message " getSymbol: invalid field: nil"

Some one can tell me what is the right syntax to specify button positions and the kind of values that I should put?

Thanks

suzan