<table border="0" cellpadding="3" cellspacing="0">
   <tr>
      <td valign="top" style="padding-top: 6px;">Name</td>
      <td>
          <input type="text" id="txt_name" />
          <div class="error" id="txt_name_error"></div>
      </td>
    </tr>
    <tr>
      <td colspan="2" align="right">
          <button id="txt_submit">Okay</button>
      </td>
   </tr>
</table>

.error {
   color: red;
}

$(document).ready(function(){
   $("#txt_name").keypress(function(event){
      if (event.which == 13) {
         $("#txt_submit").trigger("click");
      }
   });
   $("#txt_submit").click(function(){
      if ($("#txt_name").val() == ""){
        $("#txt_name_error").html("Name is required!");
        $("#txt_name").focus();
      }
      else {
        $("#txt_name_error").html("");
        alert("okay");
      }
   });
});