Getting paper reservations sent to guest accommodation is generally not a great idea, it should only ever be done with the permission of the property as if the named person is not there items get returned.

If you tell us what journeys you are making there may be alternative sources of reservations.

Details of other sources are in this thread
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105?

We did check with the hostel if it was possible and they said it was okay. The tickets are for Krakow to Budapest the 26th of September. 

But did you check that there was a person at the hostel that could pick up the reservations? Did you enter that person's name on the address?

Yes we did. But the problem is that the tickets were sent to a post office and not the hostel. And they also arrived one week early. 

Can't the person from the hostel,  who should have received the reservations, pick up the reservations from the post office?

As the reservations are sent by registered mail (REK) only the person who's name is on the address can pick up the delivery.