from the sound of it your looking to do something like when you overlap the item / be in proximity to it then you want a message to appear above it that says what the item is and tells the player to hit the E key to pick it up. that parts pretty easy and is shown in the below picture. basically on begin overlap you make sure the overlap is the character, then you allow the item to accept input from the character, and finally you set the visibility on a text render which is the message to be displayed. the end overlap does basically the same thing but hides the text.
the more in depth part of actually adding items to your inventory depends on your current implementation. it appears that your links may be backwards. with the system show you would just need a input event for the e key which sets the variable for the slow item. it looked like in the video he was just using a different int to determine what item is what so that should be pretty simple to do. so basically on E input event → cast to your player → get the variable for slot x 0> set variable.
you could always adapt that system to an array to make things easier. then you could use a setup like the bottom-most part of the picture in which i add a item to an array.
hope that helps a bit
