//Found at http://www.webmasterworld.com/forum91/4686.htm, credit being given where credit is due.
//This function added text to a textarea.
//Usage example:
//<button onclick="javascript:insertAtCursor(document.getElementById('mytext'),'Test text')">Add Test text</button> 
function insertAtCursor(myField, myValue) {
//IE support
 if (document.selection) {
  myField.focus();

//in effect we are creating a text range with zero
//length at the cursor location and replacing it
//with myValue
  sel = document.selection.createRange();
  sel.text = myValue;
 }

//Mozilla/Firefox/Netscape 7+ support
 else if (myField.selectionStart || myField.selectionStart == '0') {

//Here we get the start and end points of the
//selection. Then we create substrings up to the
//start of the selection and from the end point
//of the selection to the end of the field value.
//Then we concatenate the first substring, myValue,
//and the second substring to get the new value.
  var startPos = myField.selectionStart;
  var endPos = myField.selectionEnd;
  myField.value = myField.value.substring(0, startPos)+ myValue+ myField.value.substring(endPos, myField.value.length);
 } else {
  myField.value += myValue;
 }
}