Native Apps Driver help for C#

Error message “They type or namespace name could not be found(are you missing a using directive or an assembly reference?)”
for the below
protected static AppiumDriver _driver;

I am also trying this that works but not sure if it should be a webelement as it will need to only test the app protected static AppiumDriver _driver;

public static IMobileElement GetElement(By by)
return _driver.FindElement(by);

What am I not doing correctly?

I have also tried the below:
AppiumDriver appiumDriver = new AppiumDriver(new Uri(“http://localhost:4723/wd/hub”), driverOption);
_driver = appiumDriver;
but I get “cannot create an instance of the abstract type or interface Appium”