Nuget downloads packages, installs them to the active project in a solution, and adds the references as required. If you have no solution open, aor no current project within a solution, it has nowhere to add the package data to, and nowhere to add references.
Open a solution, run Nuget offline
] and it should add the references to the current project - the one containing the file you are viewing.
Think about it: not all projects on a machine (much less all solutions!) need all Nuget packages - so you need to be able to specify which projects do - and that's by having it open when you run Nuget.
If you have multiple projects in a solution that all need the packages, you will have to run Nuget separately for each relevant project.