There are a few possible reasons this might happen.
1. Your password needs to be less than 13 characters.
The Express PowerPoint plugin has a maximum password length of 13 characters. This is different than the website.
2. You are currently connected to a VPN.
For some users, their VPN connection causes a problem and the Express plugin is unable to connect to the PresentationPro service while they are connected. Try temporarily disconnecting from your VPN and then restart PowerPoint to see if you can then login with the Express PowerPoint plugin and apply designs.
3. You may have checked the Proxy server settings but you don't use a Proxy server.
Make sure that the email address field and the password field are filled in and nothing else and click Login. The plugin will automatically save your settings.