Going to "Package Manager Settings" under "Nuget Package Manager" under "TOOLS" brings up the "Options" Dialog box with "Nuget Package Manager" expanded. Click "Package Sources" herein. Then click the plus symbol at the upper right to add a new package source. Give it whatever name you like. The "Source" should not be a http or https URL but instead should be the file path out to your file like C:\Users\U630126\Desktop\jetbrains.annotations.8.0.5.nupkg or something similar. Once you have a solution open "Manage NuGet Packages for Solution..." will appear under "Nuget Package Manager" under "TOOLS" and this will open a dialog box when clicked and at the dialog box at the upper left under "Updates" you should be able to find your source and install stuff out of it.