The Guild Wars official site has been updated with an FAQ on skill and item unlock packs.

Q: Does my PvE character gain access to these skills?

A: Not immediately, and only if you have installed the corresponding campaign. The skills will be unlocked on your account, however, so you can purchase them from any skill trainer in that campaign. This makes buying these skills more convenient if your character has not reached all of the skill trainers yet. You can also use an elite or non-elite tome to learn any skills unlocked on your account. This will allow your PvE character to learn elite skills you have unlocked without using a Signet of Capture.

Q: So, I own Nightfall and the PvP Access Kit, but I don't own Factions. I bought the Factions Skill Unlock Pack, and I have a tome. Can my PvE character now learn a Factions skill using a tome?

A: No, you must own the related campaign or expansion associated with the unlock pack you purchased in order for your PvE characters to learn those skills.

Q: So, if my PvE character doesn't automatically learn these skills, what's in it for me?

A: Your Heroes gain immediate access to all skills (Heroes know all skills unlocked on your account, regardless of whether you are using the Heroes in PvP or PvE combat). If you own the full campaign associated with the skill unlock pack, your PvE character can also purchase any of the non-elite skills from any skill trainer from that campaign, or you can use elite and non-elite tomes to learn these skills. This may well give you access to some skills before you would encounter them by playing through the game.

Read more here.