It looks like you are using the Administrator Install of Skyline.
The Administrator Install gets installed in "C:\Program Files\Skyline" instead of being installed in your home directory.
There are a lot of problems in terms of getting external tools to work with the Administrator Install. For one, you need to run Skyline as an Administrator in order to be able to install an external tool, because it requires writing to the "C:\Program Files\Skyline" folder. Alternatively, you could give your user account access to that folder.
For people who have installed the Administrator Install of Skyline on a Windows server that is intended to be used by multiple different user accounts, external tools do not work at all. When an Administrator installs an external tool the other users do not end up seeing the menu items associated with the external tool.
But, if you are the only user on your computer, and you want to get External Tools to work with your Administrator Install, you should just give yourself permissions on that folder.
The easiest thing would probably be to use the regular installer for Skyline which puts Skyline in your home directory where you will have permissions to install External Tools: