Is there a help file in the folder that you downloaded ?
If not here a rough "what to do "guide !
If you have the PBO PAcker then it compresses a folder into PBO format ;D
Choose the folder that you want to PBO up,, open the PBO Packer and click OPEN FILE, choose the file that you want to compress. (This file has got to contain everything the mission needs to work properly)
Now click Compress or Compress File..
All done ;D
Rhys