When you try to load an executable file or add-in, a dialog displays asking you to verify its publisher. 
 
  Issue: This executable file or add-in is missing a valid digital signature. The signature ensures that the file has not been tampered with since it was released by the publisher. Before 
  Revit runs the executable or add-in, indicate whether you trust the publisher and the source from which you received the file. 
Solution: Choose the appropriate option: 
- Always Load. The file is from a source that you know and trust. In the future, when you load this file (from the same location and with the same add-in ID), it will be loaded without displaying this warning. 
  
 
- Load Once. You trust the current edition of the file and you want to load it now. If you load the file again in the future, the warning is repeated. 
  
 
- Do Not Load. Cancels the request to load the file. 
  
 
Caution: Load the file only if you trust the source from which you received it.