How do I access the user profile property from code as it returns null value ?

//This method runs when the button is clicked

you should use the references
using Microsoft.Office.Server;
using Microsoft.Office.Server.UserProfiles;

    void runQueryButton_Click(object sender, EventArgs e)
    {
        //Get the My Sites site collection, ensuring proper disposal
        using (SPSite mySitesCollection = new SPSite("http://sp/my"))
        {
            //Get the user profile manager
            SPServiceContext context = SPServiceContext.GetContext(mySitesCollection);
            UserProfileManager profileManager = new UserProfileManager(context);
            //How many user profiles are there?
            resultsLabel.Text = "There are " + profileManager.Count + " user profiles.<br />";
            //Loop through all the user profiles
            foreach (UserProfile currentProfile in profileManager)
            {
                //There is always a display name
                resultsLabel.Text += "User: " + currentProfile.DisplayName + "<br />";
                //Be careful to avoid null errors
                if (currentProfile["Department"].Value != null)
                {
                    //There is a department listed so display it
                    resultsLabel.Text += "Department: " + currentProfile["Department"].Value.ToString() + "<br /><br />";
                }
                else
                {
                    resultsLabel.Text += "Department: None<br /><br />";
                }
            }
        }
    }