Plugin translation - why it doesn't work?

Hello, I have started the Italian translation working on the .po file I found on the "Language" folder. Then I saved the partially translated file as "custompress-it_IT.po" and placed it on the same folder. However, when I try to load the plugin through WP (Italian version), all the plugin's entries are still being displayed in English.
What should I do to make CustomPress load the right .po translated file?

Thanks for your help