What he means it's that a mission .pbo file its just a folder containing several files. That folder is packed as .pbo file (like if u had done it on .rar, .zip, ...).
So u have to unpack it (unpbo it) to access to those files.
Check this topic about how to unpack: Link
U need this: ArmA Tools
(the cpbo.exe file)
But before doing anything read the topic i posted and especially my post on it.