Most likely your host has not enabled the GD or TrueType libraries in the PHP installation.
going dirextly to the page that should generate the capthcha image:
gives an internal erver error.
add the following code at line 1 of that file:
That should display the actual error. My guess is that the actual error is going to say that a function is undefined. this happens when the GD Library is not enabled on the server, the host will need to correct the problem.