I have found a solution to why the YouTube app wasn't loading on my Samsung UE40D7000 Smart TV. I found that the App loaded when the TV was connected directely with a LAN Cable but not through the wireless router (N Belkin model). In the system setup of the TV, I changed the network settings to Manual rather than Auto Detect and set the DNS server settings to 18.104.22.168. (These are the default settings for the Google server. It now works perfectly!