I am running Mac OS 10.9.2 MacBook Pro, Xamarin Studio Version 5 (Build 878)
Under network setting the proxy is configured using Automatic Proxy Configuration with a predefined Proxy.pac file.
When I click on the login and enter my credentials I get the message
"Please connect to the internet to proceed". When I then click on the ? button, it then goes off to the internet, so it is proven I do infact have an internet connection available to this machine.
I am using the Windows version of Xamarin studio via Windows 8 and can log into my account with out issue.
Created attachment 6973 [details]
If it works fine in Windows 8 on the same network, it must be an issue in the Mono class libraries.
FWIW I'm pretty sure the issue is here: https://github.com/mono/mono/blob/effa4c07ba850bedbe1ff54b2a5df281c058ebcb/mcs/class/System/System.Net/MacProxy.cs#L910