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 Instruments is in the trash bin.
Xcode 4.3 uninstalled 4.2 but it seems it did not bother installing a new version of Instruments.
sch
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.
来源：https://stackoverflow.com/questions/9541153/where-did-instruments-go-in-xcode-4-3