Hi Jamie-VV,
As it turns out, SSI support was disabled, likely due to many changes to the server code. I will take a look at this to see if I can get it working again for the next release.
That said, there are no real steps for enabling SSI.
As long as shtml is listed in the MIME settings file (Server Configurations->General Settings->MIME Settings, it is included by default), it should work by default unless you have scripting disabled.
I will get back to you as soon as I can.
Cheers,
Kevin