Update:

Where is the code for the dropdownlist controls? To get an idea of the code we need to see, below is some editorpart code that I have used before. This is an editor part that uses the PeopleEditor control to allow you to select a user that the webpart renders.

public class UserDataEditorPart : EditorPart
{
    PeopleEditor _p;

    /// <summary>
    /// Constructor for the class.
    /// </summary>
    public UserDataEditorPart()
    {
        this.Title = "UserID Information";
    }

    /// <summary>
    /// This method is called by the ToolPane object to apply property changes to the selected Web Part. 
    /// </summary>
    public override bool ApplyChanges()
    {
        // Apply property values here.
        UserDataWebPart wp1 = (UserDataWebPart)this.WebPartToEdit;
        try
        {
            wp1.StoredUserID = _p.CommaSeparatedAccounts;
            return true;
        }
        catch
        {
            return false;
        }
    }

    /// <summary>
    /// If the ApplyChanges method succeeds, this method is called by the ToolPane object
    /// to refresh the specified property values in the toolpart user interface.
    /// </summary>
    public override void SyncChanges()
    {
        // Sync with the new property changes here.
    }

    /// <summary>
    /// Render this tool part to the output parameter specified.
    /// </summary>
    /// <param name="output">The HTML writer to write out to </param>
    protected override void Render(HtmlTextWriter output)
    {
        output.Write("Domain UserID (Domain\\UserID):");
        output.Write("<br>");
        _p.RenderControl(output);
        output.Write("<br><div style='width:100%' class='UserDottedLine'></div>");
    }

    protected override void CreateChildControls()
    {
        UserDataWebPart wp1 = (UserDataWebPart)this.WebPartToEdit;
        _p = new PeopleEditor();
        _p.ID = wp1.ID + "_PeopleEditor";
        _p.AllowEmpty = true;
        _p.AllowTypeIn = true;
        _p.MultiSelect = false;
        _p.PlaceButtonsUnderEntityEditor = false;
        _p.SelectionSet = "User";
        _p.Width = Unit.Pixel(195);
        _p.CommaSeparatedAccounts = wp1.StoredUserID;
        _p.PrincipalSource = SPPrincipalSource.UserInfoList;
        Controls.Add(_p);
    }
}