Getting Proxy Digital Certificate onto my site

Hello

I have recived Proxy Digital Certificate (This cartificate is being used for sending the recipt, when a costumer has paid for their order, directly to my Financial Administration) that has been issued by my Financial Administration (Its called FURS (Financial Administration of the Republic of Slovenia)). Now I having trouble getting this certificate set up on my site. Would really appriciate the help.

Hello thanks for replying to me. Could we talk somewhere secluded about this thing?