1. Make sure there are no Visual Studio instances running.
2. Make sure you're logged in as Administrator. If not please elevate the rights for the Telerik Reporting installer prior starting the install.
3. On the "Custom Setup" step of the installer, make sure the "Visual Studio <version> Support" node is present and selected for install. If it is not present, the installer did not detect Visual Studio on your machine. This can be caused by corrupted VS installation and you should try repairing it.
4. Check whether the "new item" templates of Telerik Reporting are placed in C:\Program Files\Microsoft Visual Studio <version number>\Common7\IDE\ItemTemplates\CSharp\Reporting and C:\Program Files\Microsoft Visual Studio <version number>\Common7\IDE\ItemTemplates\VisualBasic\Reporting. Those folders should contain a file with the following naming convention Telerik_Reporting_Q1_2016_10.0.16.113_Blank/Import/Wizard.zip. If it is missing, the easiest way to recover the installation is to uninstall the product and then install it again, following points 1-3. If the item is there, check whether it has been registered in Visual Studio by looking for folder with the same name in ItemTemplatesCache folder. If such folder does not exist, open Visual Studio Command Prompt and run devenv.exe /InstallVSTemplates.

Note: We don't provide the Telerik Reporting item templates for Web Site projects. Due to some Web Site project peculiarities, the report designer has limited functionality when the report definition is created in App_Code. Thus we recommend the best practices solution structure. First to create a class library that contains your reports and then reference that class library in your Web Site project. For more information see ASP.NET Web Application Projects vs. Web Site Projects in Visual Studio and Converting a Web Site Project to a Web Application Project MSDN blogs.