K2… keep in mind K2 is nothing like a regular WordPress theme, and even has its own support forums, in part for just that reason.
All “in The Loop” modifications for K2 occur in the theloop.php template. If you want custom field data to reside with the post content, look for the “itemtext” div in the template. There are two of them to find, one for “normal” entries, and one for the asides. If anywhere else (such as part of a post’s header with the comment #, date and other *meta*), you’ll have to do some serious reading of theloop.php to decide the best locale.
In any case, visiting the K2 forums is recommended, since they best know that particular beast.