That guide assumes you have the model in XSI or other 3D model format. For the Sins models, you only have the .mesh. No program can directly add points to that.
I get around this by opening the model in notepad from the reference files, copying a hardpoint, and then open up the dev.exe to show mesh points. I can then manually change the X,Y,Z coordinates and the dev.exe will reload the model, showing the new position. Repeat until your hardpoint is where you want it.