It is not available to everyone. Only commercial license holders can download it.

If you have a commercial license then, you should also have a distribution url, a username and a password. Using these you can access the installers and the plugins.

also if u have some book regarding writing QT applications in Visual Studio, help me out
I don't think there is any book that talks specifically about Qt with Visual Studio.

Actually it is not all that difficult to integrate Qt with Visual Studio, specially with the plugin.