[Setup] Plone upgrade 2.5 -> 3.3.x

[Giuliano Castelli]

Dear all,

I need to upgrade a site from Plone 2.5 to the last version of Plone that
I guess is the 3.3.

When I go as admin to the Site Setup and after to the Zope Management
Interface and after to "portal_migration (Handles migrations to newer
Plone versions)" and after to "If you wish to upgrade to a new Plone
instance, go to migrate." I cannot find (of course) the last version of
Plone because when I installed my actual version of Plone that was the
last version of Plone.
So, how do I have to add the last vesrion of Plone in that menu so that I
can do the upgrade?

Thanks, kind regards,

Giuliano.