It will pop out! Like this!
After inserting the bearings into all of the wheels, slide them on the post and screw on the nut. You will want your nut tight enough that there is no too much wiggle room, but loose enough that the wheels spin freely. Repeat for each wheel, post and nut.
After completing your project your hands will smell like grease and be dirty. The best way I have found to get rid of all of the smell and grease is using Fast Orange hand cleaner.
NOW GO ENJOY YOUR SMOOTH ROLLING, FAST MOVING SKATES AND HIP CHECK A SKINNY GIRL!