There are probably many ways to do that, so I am going to tell you what I would do and then you see if it works for you, if it doesn't just come back here later and let us know because someone else might have a better idea.
Well if the player will fill the slots from left to right, and then use the items at the same order, all you need is a variable to tell which slot is currently being used. If variable is 0 goes to the first slot and then adds 1 to the variable, and so on. And after you use the item just destroy it and subtract 1 from the variable.
However, if the player can use the objects in any order he wants, then this means that the player can collect 3 objects, use the one at the second slot, and then you can't place the 4th item at the 3 slot again, nor at the forth.
If that is the case you will need to select your slot object and on the object settings on the left you gotta set a Instance variable. This variable will tell if the slot is being used or not. Then when the player collides with a object you can set "system>pick object by comparison" and then you compare if the value of the variable of the slot (something like this slotName.instanceVariableName) is the same as the value that tells it is empty. The game will then select a slot that is empty and move the object there.
Keep in mind, however, that this might result in objects being placed inside random slots and not in order from left to right.
For it to be in order I would do something like, set another instance variable for the slots that works as their ID so they can be organized from 1 to 5.Then and set another instance variable, but for the objects this time, that tells if the object is at a slot or not.
Then every time the player collects an object you go "system>for" set it from 1 to 5 and then compare if the object is at a slot and if the slot that has the same ID as the loop.index. This will check all the slots in order every time the player collides with something, and then once the object is in place the other checks will be false.
I don't know if I'm being clear enough, but try and work with instance variable and loops, and if you need more details just ask o/