It will be tight at first, especially if you painted the parts. In fact, they give you some shims in the box to make it tighter.
You should be able to hand push the kingpin into place, after positioning the bearing in the bottom of the opening, under the axle. BTW, you know the bearing goes so that no water can go into it from the top, don't you? The shielded part is the top of the bearing.
After driving for a while, it will loosen up ok.
Don