Thanks for the comments guys. The two current Achtung! Cthulhu books are supposed to run off the Realms of Cthulhu system, but I've found a few differences in how things work. They just finished a Kickstarter for their own GM and Player books with everything the players will need, so I would rather avoid extending Realms of Cthulhu if I can, as things will likely change when their own setting books come out.
The tome is basically book with some spells in. You can spend d4 weeks translating it and once you understand the book you have access to the spells contained within it. You don't appear (at this point in time) to need an Arcane Background to cast the spells, just Knowledge (Mythos). You have access to the spells for as long as you have the book and can use them as many times as you want assuming you have the various trappings necessary to run the ritual.
I think you may have just hit the nail on the head though... As I've got no arcane backgrounds, Hero Lab doesn't have anywhere to add the spells to (which could be our error). Maybe I need to grant everyone with Knowledge (Mythos) a special Arcane Background, even though it's not mentioned in the Achtung! Cthulhu books?
[update] Ok, I created a new Arcane Background called Mythos that basically disallows all of the other Arcane powers and bootstrapped this to the setting so all characters currently get it. This seems to have resolved the error with the book then bootstrapping my spell to the character. Thanks!
Now to work out how to add the Arcane Background when you acquire Knowledge (Mythos)! The
Bootstrap page in the wiki talks about conditional bootstrapping, but doesn't contain an example... do you know where there this is used that I can look at please?