I had this same issue on my Win10 machine.
Like paulbates said, save the dashboard & adminconsole JNLP files on your desktop. Then you need to associate those .JNLP file extensions with the Java Web Start Launcher (javaws.exe). I found my "javaws.exe" executable in my "C:\Program Files (x86)\Java\jre1.8.0_151\bin" folder.
You can make this association by right-clicking on the dashboard.jnlp file, and select "properties". then in the "Open with" option, click "change" and go find the "javaws.exe" file.
Good luck!