download %programfiles(x86)%\Microsoft SDKs\Windows\v7.0A\Bin for x64\ Signtool.exe
You need to install the Windows 10 SDK.
Visual Studio 2015 Update 1 contains it already, but it is not installed by default. You should go to Control Panel -> Programs and Features, find Microsoft Visual Studio 2015 and select "Change".
Visual Studio 2015 setup will start. Select "Modify".
In Visual Studio components list find "Universal Windows App Development Tools", open the list of sub-items and select "Windows 10 SDK (10.0.10240)".
Windows 10 SDK in VS 2015 Update 1 Setup
Of cause you can install Windows 10 SDK directly from Microsoft: https://go.microsoft.com/fwlink/?LinkID=698771
As josant already wrote - when the installation finishes you will find the SignTool.exe in the folders:
x86 -> c:\Program Files (x86)\Windows Kits\10\bin\x86
x64 -> c:\Program Files (x86)\Windows Kits\10\bin\x64\