If you are installing a Sound Set into Studio One that you have downloaded from Studio One Pro+, you will need to make sure that you are activated using Studio One Pro+ and NOT another copy of Studio One that you might have in your account. To do this, you can open Studio One and go into "Studio One" in the menu bar of the program. From the drop-down, select Studio One Activation. Check the radio button for activating using Studio One Pro+, press the "activate" button, and close/re-open Studio One. Below is a screenshot of this window...
For information on installing Sound Sets into Studio One, please take a look at the article below...