According to Wiki[specops.wikia.com], The Fubar pack contains the following (MP only):

• Double XP for a week
  
• AK-47 at Rank 1
  
• Officer Class at Rank 1
  
• Clan Accesories: Exclusive Skins/Accessories for all classes and Unique and Visually Distinct items.

You could get it by buying boxed Special Edition of the game which is probably not available anymore, but you can still unlock it for free if you follow these steps:

1. Play the game at least once so it will create configuration files (edit some class in MP)
   
2. Since the game .ini files are encrypted you will need to download this Decrypter/Encrypter made by Rick: http://mod.gib.me/specopstheline/inicrypt.zip, unpack the tool to some folder
   
3. Go to Documents\My Games\SpecOps-TheLine\SRGame\Config and drag SRGame.ini over Gibbed.SpecOpsTheLine.IniDecrypt.exe - this will decrypt the .ini
   
4. Open the SRGame.ini in any editor, scroll at the bottom and paste this after the last line:
   
   [PREORDER]
   m_fXPModifierFromPreorder=2.0
   AK47Unlock=true
   AK74Unlock=true
   OfficerClassUnlock=true
   FUBARUnlock=true
   
   
5. Save the file and drag it onto Gibbed.SpecOpsTheLine.IniEncrypt.exe to encrypt it back
   
6. Now if you run the game and go to multiplayer, you should have AK-47 unlocked!
   
7. Have fun.