There are two types of In-Game Items, both of which are purchased differently.
Steam overlay settings are required to purchase paid products.
1. How to purchase NP
In the game go to [Shop] - [NP] and select the amount that you want purchase.
2. How to purchase DLC
Go to [Steam] - [Eternal Return Store Page] - [Content For This Game]
[How can I retrieve the purchased DLC?]