If this is the only page on WCMExchange you can't take a look at, I guess a word or so on that page triggers your proxy to go into 403-mode.
I've seen that happen before on a proxy that didn't like the word 'analytics' on a page (the first 4 characters of this word triggerd the proxy to go into 403-mode).
I wonder what the word is this time, could you maybe clear that up with your system administrator?
As a workaround:
- go to wcmexchange.com
- login
- and after the login, hit [ http://www.wcmexchange.com/web/wcbservlet/com.gxwebmanager.wcmexchange.wcbdownload.servlet?wcbdownloadid=384 ]. That will take you to the download directly.