Product review (5 stars): works great on Aprilaire 400!
Purchased this for my in law's aprilaire 400 and it fits perfectly.  No instructions needed to change it - just remove the cover, pull out the holder, pull out the water tube from the holder and unclip the top of the holder from the bottom (one clip on each side).  Reverse that to put it back together and reset the 'change water pad' indicator.  One thing I wasn't sure about was whether the twin pack was really for two changes, or whether I need to use both for one change of the water pad, but as it turns out, only one is needed to change the water pad, so the twin pack is good for two changes.

I'd suggest to cycle the humidifier back on with the cover off and watch it for a few mins just to make sure you put everything back together properly and it's working as needed (it takes a while for the water to absorb into the pad when  you watch it).  Also a good time to check the float valve switches to make sure they're functioning, not stuck, etc... just lift up on the little tip of the valve switch (there are two, at the bottom of the pan) and the water should shut off .. switch should move freely.

We got timely shipping on this item.

Good luck!