Well, it's free only in the monetary sense. The true price for getting a set of new skills for Ace Combat: Assault Horizon's multiplayer mode is...your e-mail address!!! To pay that cost, you'll need to register an account on Namco Bandai's website, then hop over to their "VIP Section" to claim your free skill pack, which contains the skills (read: perks) "MG Cooldown+", "Enhanced DFM+", and "Enhanced ASM+". The section also reveals that various other bonuses are available for different Namco Bandai games, in exchange for points.
Unusual as it is for Namco NOT to charge too much money for things they know some people will pay too much money for (like those Idolm@ster aircraft skins, which incidentally are now available on the US and EU PSN stores), I'm not sure I'd be willing to fork over my email address for a paltry three perks. However, if Namco Bandai were to, say, release Idolm@ster in English, or maybe bring over the PS3 version of Tales of Vesperia, then I'd hand over my address, and maybe even money.
Ace Combat Assault Horizon DLC made available for free [Electronic Theatre]