How to open army builder?

[gellick5]

Its an unknown extension.

[Spack]

What are you trying to open? If you have a .ab file, you need to run the Army Builder software and then use the Import option from the opening game system dialog, then point to the .ab file and import. You can't just open the file, it doesn't work that way. Please see the Army Builder help system for more details on how to import files and use them.