I have just updated from 4.2 to Xcode 4.3 and \"Instruments\" is no longer there.
I tried to run it from Launchpad but was informed that this is not possible because
You can launch Instruments from within Xcode. It is no longer in the /Developer folder but in /Applications/Xcode.app/Contents/Developer.
From the menu bar, choose Xcode > Open Developer Tool > Instruments.
If your goal to profile your code with Instruments after all, choose menu "profile" from your project then Xcode launches Instruments as you probabbly expect.