My version is K2 v2.4.1.

In the file: /administrator/components/com_k2/models/extrafield.php at line 208, replace

<?php
if($active[1]){
	$output='<textarea name="K2ExtraField_'.$extraField->id.'" id="K2ExtraField_'.$extraField->id.'" rows="10" cols="40" class="k2ExtraFieldEditor">'.$active[0].'</textarea>';
}
?>

with

<?php
if($active[1]){
	$editor =& JFactory::getEditor();
	$output = $editor->display("K2ExtraField_".$extraField->id."", $active[0], '550', '400', '60', '20', false);
}
?>

Cheers!