How to use the class property in C#?

Without looking at the source code - which I assume you can do, but I don't which to download - I would assume that camera.Parameters is an array or a Dictionary, so the value inside the square brackets evaluates to an integer (and thus could be a enum) or a different type, like a string.
It's unlikely that PLCamera is an enum, as there is no implicit conversion from an enum to an int, so it's more likely that it's a class / struct of const int values, or Parameters is a Dictionary.

For a dictionary, it could be something like this:

public static class PLCamera
    {
    public static const string Width = "Width";
    ...
    }
public class ParameterBlock
    {
    public int GetValue() { return 0; }
    ...
    }
private Dictionary<string, ParameterBlock> Parameters = new Dictionary<string, ParameterBlock>();
...
    int x = Parameters[PLCamera.Width].GetValue();

If it's an enum, then it could be done like this:

private  Dictionary<PLCamera, ParameterBlock> Parameters = new Dictionary<PLCamera, ParameterBlock>();
...
    int x = Parameters[PLCamera.Width].GetValue();

enum Person { Ford, Arthur, Zaphod }

class Answer
{
    public int this[Person index]
    {
        get 
        { 
            switch (index)
            {
                case Person.Ford:
                case Person.Arthur:
                case Person.Zaphod:
                {
                    return 42; 
                }
                default:
                {
                    throw new BowlOfPetuniasException("Not again!");
                }
            }
        }
    }
}