[Plugin: Seminar Registration Manager] Search for German solution

I downloaded the program seminar registration manager and my potential clients are not able to register. When they attempt to register it reads “there was a problem processing your request: ‘The merchant login ID or password is invalid or the account is inactive.” I checked with my payment merchant “authorize.net” and they have confirmed that it is the plugin. Please help me resolve this problem. What must I do to make this plugin work properly?