Untitled

var myFrame = $("#description-frame").contents().find('body');
    myFrame.html('<textarea id="testowy_desc" rows="4" cols="50" style="border: 2px solid red;"></textarea>');

if ( $('#testowy_desc').length > 0 )
    {
        $('#testowy_desc').focus().trigger({ type : 'keypress', which : 75 });
    }
    else
    {
        alert('element not exist');
    }

<iframe id="description-frame" class="valid" frameborder="0">
 <html>
 <head>
 <meta content="text/html; charset=UTF-8" http-equiv="content-type">
 <style>body {background: #FFFFFF;margin: 0px;padding: 0px;text-align:left;font-family: Tahoma, Verdana, Arial, Helvetica, sans-serif; font-size: 0.875em;}.desc{color:gray;} body.error { background-color: #EFDBD6;} body.valid { background-color: #F0FFC1; color : #468847; } P {margin-top:0;margin-bottom:0}a{text-decoration: none;color:#000}
 </style>
 </head>
 <body id="rte" class="valid">
     to jest testowy opis
 </p>
 </body>
 </html>
 </iframe>