If you extract the files and open the SkinInfo.xml you’ll find:
<MsgVersion Major="15" Minor="4" />
As well as:
<Version Major="15" Minor="4" />
Under the Restrictions element all 16 times through the skin.
So if you change the first to
<MsgVersion Major="16" Minor="4" />
and all 16 instances of the second to:
<Version Major="16" Minor="4" />
16.4 being the Messenger 2012 version.
You can then re-zip the files and change the file extension to .plsk. Plus will now accept the skin fully and try to load it.
Then it will crash on signing in. If you get into this state, you’ll need to head over into the registry to the following key:
HKEY_CURRENT_USER\SOFTWARE\Yuna Software\Messenger Plus!\Windows Live Messenger\GlobalSettings\Skins
and delete the CurrentSkin value.
Now this skin claims Messenger 2012 compatibility and it does work, partially. It seems to work correctly as long as Messenger is in full view (this is the view with the social view and the contact list on the side), but in compact view, it either freezes or crashes. Whether or not this is an oversight of this skin specifically or a Plus incompatibility with Messenger 2012 I’m not entirely sure.
All that said, if you’re serious about skinning, these tutorials are good start and plenty of knowledge in the old Messenger Plus forum.