I had the same issue but installing the Windows 8.1 SDK as per Catquatwa’s answer did not work for me (signtool.exe was still missing from C:\Program Files (x86)\Microsoft SDKs\Windows\vX\Bin).
I stumbled across this solution: http://www.benedykt.net/2015/08/12/missing-signtool-exe-w-visual-studio-2015/
Basically, for VS 2015, this would be:
- Open Programs and Features
- Select “Microsoft Visual Studio 2015” and click “Change”
- Press “Modify” to progress to Features options
- Select “Windows and Web Development”, then tick “ClickOnce Publishing Tools” for installation
- Then “Next” and then “Update”