The different versions of Poser have some different bugs, but injection should work with all of them (actually P5 requires marginally more setting up, as you have to put the folder with the injectable data in the right Runtime). Create a new scene (Cmd/Ctrl-n). Is it empty? If not, delete any figures and either set that as your prefered state (page 26 of manual) and do a new scene again or just save the empty scene, close it and then reopen it. Load M3 from the Figure library (if you've only just bought it you should have all the updates pre-applied, if it's older go to DAZ and get the updaters for the base figure and body morphs, though injection worked fine on the pre-updated version). Go to the Pose library, choose the !M3 Full-Body INJ folder, double click on Muscular1. Select M3's body - has a dial appeared at the top of the stack of parameters? If not try selecting a body part, then go back to the Body. If nothing is there you have a bad download or install, if the dial is there just turn it, or type in a value, to apply whatever degree of the morph you want.