If you want the machine model to be visible in the interface use this.

open the .fff file with notepad or similar and edit the line

C:\Users\USERNAME\AppData\Local\Simplify3D\S3D-Software\cr-10 frame-S5.stl so USERNAME is replaced so that it matches your own username

So if your username is johndoe it should look like this

C:\Users\johndoe\AppData\Local\Simplify3D\S3D-Software\cr-10 frame-S5.stl then copy the stl file to the correct dir on your harddrive. e.g. if your username is johndoe

C:\Users\johndoe\AppData\Local\Simplify3D\S3D-Software

Then open simplify3D and import the fff file

I found the help on how to get it working here https://forum.simplify3d.com/viewtopic.php?f=9&t=3758&p=16462#p16462 https://forum.simplify3d.com/viewtopic.php?t=2803

And the stl file is modified from a model of a 300 model that I can't seem to find again.

Though probably from google warehouse as there is a ton of versions there.