What credentials should I use?

Synchronizer uses Windows authentication. You should use Windows User credentials from the machine where Synchronizer is installed.

Note that user accounts without password cannot be used to access Synchronizer.

Domain field should be left empty for local users.

Still cannot log in?

In order to log in to Synchronizer, the Windows user must have Read and Write permissions to the file located on the machine where the Synchronizer is installed:
C:\inetpub\wwwroot\tfs-jira-synchronizer\App_Data\security\administration.acl

Visit TFS4JIRA online documentation for more help.