However, this is a new premium member feature, so if you aren't seeing it, that's probably why. Check out your Favorite Searches. Assuming you have at least one in which you are receiving notifications, the new option should be readily apparently. You can now specify whether you want to be notified of "new boxes" or of "new and modified boxes."
AQ doesn't really track what is modified on a box listing, so it won't tell you that. It just keeps track of when the last time a box was modified, for whatever reason, and if one matches your search and the last modified date is more recent than the last time you got notifications, you'll be notified of the changed listing.
But also keep in mind--you'll only be notified of the changed listing if the box still matches your search! For instance, if someone changes a box status from active to retired, you probably will NOT get notified of that change since most searches are set to hide retired boxes. In this case, the change causes the box to no longer match your search and therefore the notification of the change won't happen.
On the flip side, however, a box that had been retired but was replaced might have its status changed to active. The old listing might not have matched your search since it was retired, but the modified listing now will match your search so you will get notification of that change.
So this feature is not a replacement for the 'watch' feature. They work very differently! But keep in mind that the results may not be as intuitive as you might first expect. It does have some advantages, though. It can notify you whenever someone updates the clues on their box, or when retired boxes are brought out of retirement. Just remember, though--it will probably not notify you if a box is retired (unless you explicitly mark your search to include retired boxes), nor if you suddenly meet a restriction for a box that you never met previously.