Since Windows 8, Microsoft moved makecert.exe from the Visual Studio installation to the Windows SDK. For Windows 10, get the SDK here.
Then open a developer command prompt and issue this command:
makecert.exe -sv myAzureAutomationCert.pvk -n "CN=My Azure Automation Certificate" myAzureAutomationCert.cer
Then in the Azure portal, use the instructions in this azure.com documentation page to finish the process.
Cheers!
