Marketpress doesn't allow me to upload a product file

When I try to upload a digital product, I receive the following error:

[code]
Forbidden

You don't have permission to access /wp-admin/media-upload.php on this server.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
[/code]

Have I misconfigured something?

Thanks!

Edward