I know that I can unlock the new machete skins by linking my Ubisoft (now called Uplay) account to the game and entering my CD-key. Apparently I can also unlock the "Predecessor" extra missions, that were supposed to be available only to those who pre-ordered the game from certain merchants (one merchant offered the Le Francais, another offered The Bolivian, etc), just by entering my CD-key in the "Additional Content" box a second time. Once unlocked they would appear as "Predecessor Tapes" right below "Jackal Tapes".

The problem now is that when I try to enter the key into this box, it tells me I need to be logged in first. The only place I can log in from the Main Menu is in the Multiplayer page, and to get from there to the Additional Content page is to click the back buttons, which apparently logs me out because entering my key only results in a "You need to be logged in" message.

Take a look at this thread: http://forums.steamgames.com/forums/showthread.php?p=20431626

Maybe something there will help you.

Yes, and I've read that thread when I posted this thread. As you can see, the author of that thread posted the link, probably without reading my post all the way.

No, the problem is that I can't enter the CD Key into the "Additional Content" box a second time without getting a "You must be logged in first" error. I've already linked my Ubisoft account and entered the CD key once, and redeemed the extra machete skins.

The thread you've linked me to doesn't explain how to log back in to enter the cd key to unlock the predecessor tapes.

That sounds like the same bug I had when trying to redeem the free Avatar DLC. Despite numerous tickets submitted to Ubisoft support and claims that they had fixed it, it never did work.

See, I had thought that the "Predecessor" tapes were only available to "prepurchase" customers, and even then from certain merchants (no one merchant gave a code for all tape sets). The unlock codes on cheat sites weren't supposed to work because they were unique keys that would be checked against an online set, since Far Cry 2 had to be connected to redeem any online content. Now, apparently, the unique pre-purchase content has been patched to be available to all who buy it on Steam.

I'm going to try deleting my multiplayer profile and making a new one, since it uses the same Username/Password as my Uplay (formerly Ubisoft.com) account. It's probably something stupid like that.

Ok, I figured out how to get the game to accept my CD Key again. Apparently I have to launch the Map Editor first, exit, then launch the game proper. Then the Additional Content box will accept my CD Key, and then say "The Missions have been unlocked".

The problem now is that when I quit and leave, and then come back, the menu option "Predecessor Tapes" (below Jackal Tapes) is gone, and I have to repeat the whole process of unlocking them.