Drag the dlc file into horizon and press “save to device”.
Most of the DLCs in the link I gave you comes in parts (part 1, part 2, part 3 etc) 
Here’s a quote from @SteveWonda explaining how to add them:
Also, you can not drag rar/zip containers into horizon.
You should also take a look at this thread Aden34's DRM Free Thread. NEW LINKS! (Bioshock Infinite)