Exactly, you need a VM app to run PM Injector. Try NoxPlayer, personally that worked for me, but as far as I know, the author tested it on LDPlayer as well. Once you install the VM app, make sure you're using 64bit instance and your root is enabled. From that point, it's really easy - just run Whispers and PM Injector (grant it superuser privileges), wait until Whispers is on the home screen and set up PM Injector as mentioned in the first post. Hope this helps - if not, let me know, I'll try to explain further. :)
EDIT: It also depends if your on your phone or PC. For Android, I would suggest F1 VM (VMOS is not really reliable, unfortunately), if you're on PC, try Nox/LD. Some users were in luck with MEmu as well.