string error = null;
TableNode tn = this.xamRichTextEditor1.Document.InsertTable(0, 3, 2, out error);
if (error == null)
{
    int cRow = 1;
    foreach (TableRowNode trn in tn.ChildNodes)
    {
        int cCell = 1;
        foreach (TableCellNode tcn in trn.ChildNodes)
        {
            ParagraphNode pn = tcn.ChildNodes[0] as ParagraphNode;
            pn.SetText(string.Format("Row {0}, Cell {1}", cRow, cCell));
            cCell++;
        }
        cRow++;
    }
}