If your tax certificate has expired, apply for a new one in eDavki.
You download the tax certificate on your computer and download it to MetaKocka
Certificate transfer - a button to select the certificate is shown - click on it

Select/download the certificate that you accepted from eDavki, for signature and transfering information to FURS. (http://www.datoteke.fu.gov.si/eDavki/PrevzemNamenskegaDigitalnegaPotrdila.pdf). At this dialog for certificate download use the same password as you enetered at accepting from FURS.