The error message you are seeing, 'server unreachable', means that your VPN client on your device can't reach the server.
There are several possible reasons for this such as a just a sticky connection, you are too far away from the router, you are behind a firewall or your Internet Service Provider's DNS server (the server that changes web addresses to numbers) is unable to accept the large DNS response from our mobile servers.
First make sure you have a strong signal, try disconnecting and reconnecting several times to ensure it is not just a sticky connection. Then if practical try to connect through another internet connection eg if you are using a router try your mobile connection or move to a different location. Finally, if you are using a IPad/iPhone/iPod, we suggesting that you make a change to the wireless settings on your iPad/iPhone/iPod so that you use Googles DNS servers instead of the ones from your ISP.
There is an excellent guide on how to do this at : http://www.macinstruct.com/node/452
The above guide uses an iPad as reference but the same procedure applies to iPhones and iPods too.