The scrubber pad has a hole in the center like a donut, you just have to dig for it a little. You slip it over that black tube in the center of the top part of the catch can, then screw the reservoir body onto the top part. You'll have to keep pushing the fibers in to clear the threads, but you'll eventually get it started and can screw it together. Having the ball valve to drain is a huge convenience.
I have the same catch can and mounted it next to the alternator. Easy to reach and plenty of space underneath for a drain cup.