Accessing a class

Hi,
I have this methode on my main form:

        private void MainForm_Load(object sender, EventArgs e)
        {
            ClearFormFields();
            xmlDocument = new System.Xml.XmlDataDocument();
            xmlDocument.Load(strXmlDocument);
            ????????. XPathFilter = "attribute::id";  <=======================

How to access the methode XPathFilter in another class from the main form.

The class looks like this:

namespace DemoCatalogue
{
    class ProdCatalogue
    {
        public string XPathFilter
        {
            get
            {
                return xpath_filter;
            }
            set
            {
                xpath_filter = value;
            }
        }

    }
}

Who can ehelp me?

Peter