You can load up 7 datapaks maximum (I am not 100% sure, but I tried to load more and it didn't work)

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.

Example: C:\Users\User\Desktop\Datapaks\FPS_Boost.pak=1