With the release of Dynamics 365 (online), version 9.0, there are a lot of changes in our developer documentation:
No more monolithic SDK download : Getting the assemblies and tools you need
SDK assemblies and tools will be distributed only via NuGet. We will provide a script that will allow you to download the assemblies and tools from NuGet.

You can follow below steps to download Nuget Packages using Visual studio

Step 1 Right Click on to your visual studio project and click on to manage NuGet Packages

Step 2 Search for "Microsoft.CrmSdk.XrmTooling.PluginRegistrationTool" and click on install

Click on I accept

Step 3 Navigate to the package directory of that project

Step 4 Search the folder Microsoft.CrmSdk.XrmTooling.PluginRegistrationTool.X.0.0.X

Done!! now you will be having your latest Plugin registration Tool

Sample PRT Reference for quick start  : Microsoft.CrmSdk.XrmTooling.PluginRegistrationTool.

In the same way you can download any of the NuGet assemblies and tools

Hope it helps !!

Happy CRMing

SOURCE : mscrm.com