As far as I know the soundset will work on a Mac, and once you get past the installation stuff it should work pretty much the same way as described in the rest of my not-so-good documentation. 

I'm not sure 'exactly' where on a mac one should place the xml files for soundsets, but if you can find it and drop a copy of my xml file in there it should work.  I believe they typically go in (Location that makes it available for all user accounts on the Mac):

Macintosh HD/Library/Application Support/Avid/Sibelius/Sounds

It might not be in this 'exact location', and there might be different ones for various versions of Sibelius that might have been installed on your system at some point.  Once you find it, you'll probably notice a lot of soundsets already there for things like GPO4, JABB3, General MIDI, Yamaha XM, etc.  If you have more than one version of Sibelius and/or multiple "Sibelius (version)/Sounds" directories, drag a copy in in the "Sounds" directory of them all.

I think I once read that "Library" is usually a hidden directory on Macs, so if you don't know how to see those on a Mac check out this link:

http://www.macworld.co.uk/how-to/mac-software/how-show-hidden-files-library-folder-mac-3520878/

Here are some links on the topic of installing Sibelius soundsets on a Mac.  While this link stops with instructions for Sibelius 7...it should be the same for Sibelius 8.  If you add it in the user location designated in this link, it's only going to work for that user, so if you need it to work in multiple accounts, see if you can locate the default Library directory for Sibelius Sounds discussed above.

http://avid.force.com/pkb/articles/en_US/How_To/How-to-install-a-Sibelius-sound-set?retURL=%2Fpkb%2Farticles%2Fen_US%2Fhow_to%2FHow-to-install-a-Sibelius-sound-set&popup=true