The beta releases of Charles introduce new functionality for testing. Beta releases will generally be announced and documented on my blog.
The latest beta version of Charles is 3.10b6. Please check that this is newer than the latest release version (3.9.3) before downloading.
Note for 3.10b5
Note that 3.10b5 generates a new SSL certificate for each installation of Charles. You can export the certificate using the Help menu. For other devices, such as mobile devices, setup their proxy to Charles and then visit http://ssl.charles/ to download the current SSL certificate.