Hi, I ran into this type of problem myself and was about to drop my subcription to rockettheme.
Than I found out a way to solve this problem: As you can't create a new content element because your inside a menu item, here is what I found:
Go into administrator/components/com_joomfish/contentelements/menu.xml
and change the line :
<field type="params" name="params" translate="1">params</field>
to
<field type="textarea" name="params" translate="1">params</field>
You'll then be able to see a box to let you change the subtitle in joomfish.
cheers,
[edit: I corrected my mistake Kat is pointing out further down]