Hmm I've follow this suggestion to the letter & installed with: sudo apt-get install python3-smbus.
However it doesn't do a thing for me, the problem still persists with my Explorer HAT Pro, in as much as I still get the same warning as Dan doe's, "Python-smbus isn't installed" when using Python 3.
For the record I'm using the latest version of Raspbian Noobs: Version:1.4.2, Release date:2015-09-24 - Any suggestions would be gratefully received?
Personally IMO the Git could do with a re-write to make it a little easier for youngster's & newbies to follow, with clearer instructions on how to install the examples library & instructions on the basic usage of the HAT with perhaps examples including a picture or two, showing something basic like lighting an led or connecting a motor to the to the hat.
If you were a youngster who'd just received this as a present or used their pocket money to purchase, I think you'd struggle to use it with it's current documentation. Wading through support forums isn't something you have should do for a retail item.
Cheers
Mark