I installed the latest version of NinjaTrader v184.108.40.206 and rebuilt the Alarm Manager, it now works ok.
Please download again the alarm manager from your account downloads and test.
When you install the AddOn make sure you also include all of the files that are required, if you miss one then you will get the error above. if it still does not work for you tell me the operating system you are using and I will reopen this ticket.
This product requires Windows 10 and above.