In order to open it in the editor, just unpack the PBO file using a PBO unpacker like ExtractPBO. You can get it here:
https://dev-heaven.net/projects/mikero-pbodll/files
Pick ExtractPboGui2.00.rar.
Open it, then select your PBO file. Select a destination (probably .../documents/Arma 3 Alpha/missions/) and click extract.