Calibre (Library) button is missing

Hi folks,
i just installed calibre 8.3 (windows 10 Home). I am trying to import the calibre-data files i exported from another computer. Unfortunately, on this install, the Calibre Libray button is missing, so i cannot import the whole datas from my former computer.

Is there a way to get it back ? I'm using v 8.3 on both computers.

Thanks in advance.

Niko