First of all, please note I'm using ResEx 1.5 beta on Win XP SP3 (Italian).
Here is what I did to localize my resources.
1) Open the original .resx file.
2) Choose Tools > Add Culture > Italian (Italia).
3) Save the resource file as [filename].it-IT.resx (or did I change the name in Explorer? - can't remember).
4) With the Base Value column on the left, and the it-IT Value column on the right (both after the Key and Base Comm columns) I started translating the it-IT Value column.
5) After saving, closing and reopening the file, all Base Values where 'copied' from the it-IT Value column.
Do you see where the mistake could be?