You can't easily use the guitar that came with the game, because it's a scripted object with its own interactions that don't work with poses. You need a simple prop.
If a prop guitar came with the poses, it may be an accessory in CAS. If you go to the accessory tab when you are planning an outfit, you should see a custom icon for the guitar, or a weird, blank looking body part (if the creator didn't bother with making an icon.) It might also be in the deco/misc category or something like that, if it's an item rather than a CAS part. It would have a custom content icon on it.
If no guitar came with the set, and if the creator didn't tell you exactly which one he/she used and where to find it, you can find some by googling 'sims 3 guitar accessory.' Be careful where you download from, and look for other people to have commented that it works well before you download it, or you might get garbage. (You might get garbage anyway, as with any cc, but you probably know that.)
It is theoretically possible to also use the guitar that came with the game as a prop, but you need to download some other stuff to manipulate it, and you also need to do some photoshopping to get rid of the stand, like I did in
this pic. But that is a lot of work, and you typically just want an accessory of some kind.
Hope that helps.
