If the new object is there, though, theres something very wrong with your game. Youll need to find it and remove Pose Player from there. But that is a lot of work, and you typically just want an accessory of some kind. If it is not, then that would be further proof that you are working with the wrong Mods folder and theres another The Sims 3 folder somewhere on your computer that the game is reading. 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. (You might get garbage anyway, as with any cc, but you probably know that.) 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.

It would have a custom content icon on it.

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. If you have questions please ask them in the support thread If you would rather use Blender to make poses with the tutorials for that are at Simlogical.

If a prop guitar came with the poses, it may be an accessory in CAS. See below for instructions on making a pose using Milkshape. 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.

The sims 3 pose player