Hi. The class that your compiler doesn't recognise should have a red squigly line under it. Mouse over that class and a light bulb should appear with a triangle. Click the triangle and it will offer to add the relevant using statement if that is what is needed. If installing a Nuget package is needed it may offer to install it and add the using statement.

If that doesn't work then right click your project or solution in the Solution Explorer and click Manage Nuget Packages... in the menu to open the Package Manager. On the Browse tab you can search for public Nuget packages, search for the package you need and click on it in the list. On the right you will now see the projects in your solution, click the checkbox next to the project you need to install it in and then click Install.

Clean & rebuild your solution (not always necessary but sometimes helps so I usually do it after installing a Nuget package) then try the light bulb again to add the required using statement.