Find custom content in Sims4Studio

If you want to re-clone the item you need to place the .package in Documents/Sims 4 Studio/Mods in order to see it in the program. You need to place it in your Sims4/Mods folder too.

In case you want to edit the mattresses overriding the existing just edit it with the "My Projects" button. Have in mind that if you do that and share the .package you edited people that have both (The unedited and the edited) will not be able to get both in game. So I think you're better cloning it.