1. Drag dide_mod.ini, dsound.dll in your Dying Light directory.

2. Create a folder anywhere on your computer, desktop as an example.

3. Move datapaks that you want to play with in multiplayer in that folder.

4. Open dide_mod.ini in your Dying Light directory, after line "[CustomPak]" paste the path to the datapak that you want to load in multiplayer with =1 at the end.
How to copy and paste the path: https://prnt.sc/trdhlq
Example of pasting your path with =1 at the end: https://prnt.sc/trdilh
Note: you can load up to 10 Datapaks, just paste another path to the datapak with =1 in the end on the other line. Screen: https://prnt.sc/trdjv8