Newbie - I got a solution that was missing an explanation on how to include a reference - please help me

I got the following solution code, and the provider did not explain how I needed to go about properly adding a namespace (reference?  Using?  something else?).  The provider's last comment is immediately below, followed by the code solution he provided.  I get 40-50 errors when I try to add a using statement for System.Runtime.InteropServices namespace, and my original question was closed after I got a nastygram from the moderator on my still having my question open.  (Sorry for all the background there) - can SOMEONE please detail what I need to do to get the solution code below to work properly in C#?

(provider's last comment) Partly my fault--I forgot to tell you to include the System.Runtime.InteropServices namespace. This is needed for the DllImport statements  :)

(following is provided solution code that I cannot get working)
public partial class Form1 : Form
{
    [DllImport("advapi32.dll", CharSet = CharSet.Unicode, ExactSpelling = true, EntryPoint = "RegOpenKeyExW", CallingConvention = CallingConvention.StdCall)]
    public static extern int RegOpenKeyEx(UIntPtr hKey, string lpSubKey, int ulOptions, int samDesired, out UIntPtr phkResult);

    [DllImport("advapi32.dll", CharSet = CharSet.Unicode, ExactSpelling = true, EntryPoint = "RegLoadMUIStringW", CallingConvention = CallingConvention.StdCall)]
    internal static extern int RegLoadMUIString(UIntPtr hKey, string pszValue, StringBuilder pszOutBuf, int cbOutBuf, out int pcbData, uint Flags, string pszDirectory);

    [DllImport("advapi32.dll", ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
    public static extern int RegCloseKey(UIntPtr hKey);


    public static UIntPtr HKEY_LOCAL_MACHINE = new UIntPtr(0x80000002u);
    public static UIntPtr HKEY_CURRENT_USER = new UIntPtr(0x80000001u);
    public const int READKEY = 131097;

    private void button1_Click(object sender, EventArgs e)
    {
        const int BUFFSIZE = 1024;
        UIntPtr key;
        StringBuilder buffer = new StringBuilder(1024);
        int pcbData;

        long result = RegOpenKeyEx(HKEY_LOCAL_MACHINE, @"SYSTEM\CurrentControlSet\Control\Class\{36FC9E60-C465-11CF-8056-444553540000}", 0, READKEY, out key);
        result = RegLoadMUIString(key, "ClassDesc", buffer, BUFFSIZE, out pcbData, 0, null);
        RegCloseKey(key);
    }

    public Form1()
    {
        InitializeComponent();
    }
}