It seems you have taken label instead of textbox, anyway below is working code.
function addRow(id) {
var label = ($("#txt1").attr("value"));
var label2 = ($("#txt2").attr("value"));
var tbody = document.getElementById(id).getElementsByTagName("tbody")[0];
var row = document.createElement("tr");
var data1 = document.createElement("td");
data1.appendChild(document.createTextNode(label));
var data2 = document.createElement("td");
data2.appendChild(document.createTextNode(label2));
row.appendChild(data1);
row.appendChild(data2);
tbody.appendChild(row);
}