C:\Plugins does not exist, unless you create it.
Open Explorer, select C: and add a new folder, e.g. with a right-click.
From the Guide:
Next, copy the “NegativeLabPro.lrplugin” folder into a folder
that is on your root hard drive. I strongly recommend something
like C:\Plugins – the placement of this matters because
of the way folder permissions work in Windows.
Do NOT try to place this inside an existing Lightroom app folder.
Placing this folder into C:\Plugins will
eliminate 90% of installation issues.
The guide simply assumes that everyone knows how to create a new directory…