If you have it setup the same way on your mac as you do on the pc where it is working I'm not sure why you would not be able to see the captcha properly. When you preview the page locally on your mac do a view source of the page and post back with what you get. If we can't see the live page this will be the next best thing. When we can view the source we should have a better idea of the actual values that are being used for the captcha and not just what they have been set to. I imagine there will be some issue involving the values used in the source of the page.