/templates/MYTEMPLATENAME/html/com_k2/templates/default/item.php is the file you should edit if you are not using any other K2 subtemplates.

"Insert your normal attachment code here" ??

This means the default code already present in the default template.

in fact I have now tried to rename the item.php and attachments work on the site ...
then how can I find the right file that I have to edit?

If the are not other K2 sub-templates, then something is blocking the default overriding method.

It could be a 3rd party K2 plugin or the template's framework (I would start by asking the template's developer.)

Krikor wrote: (I would start by asking the template's developer.)

www.gavick.com/joomla-templates/creativity

Exactly, you need to ask them about the available K2 overrides and explain this issue.