Ball nuts and rail bearings usually have a tube somewhere on the outside. Balls are loaded through that tube. A big mistake is to try to put too many balls in: they can jam up. The photo below shows a stripped down ball nut:
Attachment 395714
There are two bits of pressed metal shaped like drawer handles in the RH plastic tub - along with all the balls. There is also a wide saddle below them: that holds the tube bits in place.
Does not hurt to add a tiny bit of grease to the balls as they go in: keeps them in place if nothing else.

Cheers
Roger