Did you copy the whole template across and replace the old template changes? Did you make sure you have the privatemessageid variable as hidden included?
Make sure you have this:
Code:
<input type="hidden" name="privatemessageid" value="$privatemessageid">
in your privatemsg template in the quick pm reply box form - use the template that I provided.
It does work - trust me.

You just need to give the delete function the private message id.