Firsst of alll, unmount everything
1) First exctract Disc 1 and 2 into a folder
2) Make a new folder called cod mw3
3)Copy all the exctracted files from Disc 1 into another folder
3) Copy all the extracted files from Disc 2 into the same folder as you did for Disc 1
4) then you should have everything in that folder
5) the click SETUP and wait until it asks for Disc 2
6) Then using PowerISO, click on the small icon and chose the first option "mount image to drive" and find Disc 2
This is how i solved mine and it works perfectly fine
Hopefully this helped you guys