PocketBook 623 system folder nowhere to be found

Hi,
Can anyone help?

I wanted to install dictionaries in my PocketBook 603 system folder, but it is nowhere to be seen. Even opening hidden folders, as recommended doesn't make it appear and when I tried to create a new one I was told 'system' folder already exists, of course, but where?

I've never come across this problem anywhere before, and indeed, my old PocketBook 903 had no similar problems.

Hope someone can help..........................

Cheers