El getElementById te trae solo un elemento, para los radios debes de usar getElementsByName o by TagName
Código Javascript
:
Ver original<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-US" lang="en-US">
<head>
<title>New document</title>
<meta http-equiv="content-type" content="text/html; charset=iso-8859-1" />
<script language="javascript" type="text/javascript">
function Mostrar (id,elem) {
var t = "elemen" +id;
var elemento = document.getElementById(t);
elemento.value= elem;
}
</script>
</head>
<body>
<form id="f1">
<input type="radio" name="sexo" id="sexo1" value="Mujer" onclick="javacript:Mostrar(2,document.getElementsByName('sexo')[0].value)" />
<input type="radio" name="sexo" id="sexo2" value="Hombre" onclick="javacript:Mostrar(2,document.getElementsByName('sexo')[1].value)"/>
<br>
<input type="text" id="elemen2" value="" />
</form>
</body>
Saludos.