I'm sorry I mean tag <table>. I deleted it by mistake.
_annotation = new NRoundedRectangularCallout();
_annotation.TextStyle.TextFormat = TextFormat.XML;
string ret = "";
ret += ("<table><tr><td>" + dr["SAMPLEBY_NAME"].ToString() + "</td>") ;
ret += ("<td>" + dr["ENTERBY_NAME"].ToString() + "</td>");
.
.
.
_annotation.Text = ret;