Short answer (since I wont use this component anymore)
in the template file mentioned below (default.php)
insert this code at the begining
<?php
$user=&JFactory::getUser();
?>
then find the input generation @line 247 :
<input type="text" name="<?php echo $field->name; ?>" value="<?php
if ($field->name=="user") {echo $user->name;} else if(empty($this->post) ==false) {echo $this->post[$field->name];} else {echo $field->t_initvalueT;} ?>" class="<?php echo $validationclass; ?> inputbox ckCSSinput <?php if ($field->custominfo != "") {echo "ckform_tooltip".$this->ckforms->id." ckCSSTip ";}?> <?php echo $field->fieldCSSclass; ?>" <?php if ($field->readonly == "1") {echo ' readonly="true"';} ?> title="<?php echo $field->custominfo; ?>" />
of course you form must contain a field called "user" to be prefilled
