ArkivDigital is on the Premium Family History Websites list on the FamilySearch Center Services Portal. Use it from that list. (https://fhc.familysearch.org). After you click ArkivDigital, the site can default to Swedish. To switch to English, click the second drop-down menu from the right (Svenska), and click English. If you see a prompt for a username and password, click the Log in button.
The old ArkivDigital desktop program (previously available in the FHC Application Finder on FHC computers) no longer returns search results, and is no longer supported. If the program is installed (shortcut on desktop, or in the Start menu), uninstall it, and use the link in the premium websites list instead.