شما در تابع سازنده فرمتون. همون بخشی که نوشته InitializeComponent(); این کد رو اضافه کن.
RadMultiColumnComboBoxElement multiColumnComboElement = this.cmbOurCountry.MultiColumnComboBoxElement;
multiColumnComboElement.DisplayMember = "name";
multiColumnComboElement.ValueMember = "ID";
multiColumnComboElement.DropDownSizingMode = SizingMode.RightBottom;
multiColumnComboElement.DropDownMinSize = new Size(400, 100);
multiColumnComboElement.EditorControl.MasterTempla te.AutoGenerateColumns = false;
GridViewTextBoxColumn column = new GridViewTextBoxColumn("name");
column = new GridViewTextBoxColumn("name");
column.HeaderText = "نام";
column.Width = 100;
column.HeaderTextAlignment = ContentAlignment.MiddleCenter;
multiColumnComboElement.Columns.Add( column);