The EOS 500D has no facility to communicate with your flash gun. It can only 'talk' to Canon Speedlite flash units (or possibly Speedlite compatible flash units, if they exist).
That's confirmed both by your camera's user manual
http://files.canon-europe.com/files/soft33601/Manual/EOS500D_EN_Flat.pdf
and this description of your flash unit (which specifically states that it's NOT compatible with Canon EOS models which aren't listed on that page:
http://www.heritagecameras.co.uk/bounce-zoom-e-ttl-flash-for-canon-eos-d30-d60-5d-10d-20d-30d-300d-350d-400d-digital-some-35mm-567-p.asp
I can't find an instruction manual for your flash unit but the photographs of it seem to suggest that there are no controls for overriding the settings provided by the camera. Thus (given that your camera is incapable of providing those settings) it seems that you're trying to do the impossible.
Sorry!