Note that before installing the NuGet packages, you need to create a Rider project of the desired type. The NuGet packages get installed "into" the project.

This text assumes that you are using JetBrains Rider 2018.1. With other versions, the procedures may be different, but should be reasonably similar.

In order to install a OPC Data Client or OPC Wizard NuGet package and reference it from your project

1. Switch to the NuGet window in the Rider; if it is not visible, use View -> Tool Windows -> NuGet (Alt+7) command from the menu.
2. In the NuGet window, switch to the Packages tab.
3. Make sure that package sources (the second control to the right of the search box) include “nuget.org”.        
4. For OPC Data Client, type QuickOpc into the search box. For OPC Wizard, type OpcWizard into the search box.
5. Select the package you are interested in under Available Packages list in the left pane of the window. Its details appear in the pane next (right) to it.
6. If needed, select a specific version of the package in the upper part of the right pane.
7. In the right pane, press the green + button next to the name of your project.
8. Press the Yes button when asked to confirm the installation of the package.
9. The NuGet client will now update your project as needed. Among other things, it will download and copy over the OPC Data Client o OPC Wizard assemblies, and reference them.
10. You can now start using the types from the referenced OPC Data Client or OPC Wizard assemblies.