Windoze or OSX? In either case (I think) open iTunes, got to 'apps' and 'right click' on the MasterFader icon. In OSX this will give you a "show in finder" option and you can copy and paste the app into another folder/on the desktop/into Dropbox or something. In windows the option will probably be "show in windows explorer" then you can do the same to put a copy of your app safe.
The other way is to search for the app, iTunes apps have a .ipa file extension so a search for 'Master Fader 2.1.1.ipa' (or just 'Master Fader .ipa, leave spaces instead of the version number, will find any versions on your computer) should find it in finder, windows explorer or my computer.
If you have already updated to MF3 you may still have a copy of MF2 in your trash/recycle bin.