Xcode ceased to ship Shark in Xcode 4 (released March 14, 2011), so this documentation is largely obsolete. See (and please contribute to) the Instruments documentation instead. (Instruments replaced Shark in Xcode.)
Shark is now hard to get. It's still available as part of CHUD (Mar 23, 2009) in the developer downloads.
You may be able to get shark from here:
Click on developer tools, which should bring you to here:
Where you will find CHUD 4.6.2, which includes Shark 4.6.2 (227.9).
Unfortunately, this version does not seem to work with 64-bit kernels.
With new Macs it should come on the OS X DVD. This version, 4.7.3 (383), worked at first, but now no longer works:
11-05-31 3:52:50 PM [0x0-0x18e18e].com.apple.chud.modularshark WARNING: Length of the text section of com.apple.driver.AGPM 100.12.31 was reported to be 0xc000 (c pages) from the kernel, but we're using <VMUSymbolicator: 0x19f49350> (<VMUArchitecture: 0x19f47ca0> 0x1000007,0x3 64b - LittleEndian) /System/Library/Extensions/AppleGraphicsPowerManagement.kext/Contents/MacOS/AppleGraphicsPowerManagement that thinks the length is 0x9000 (9 pages). [-[KmodSymbolLookup _stretchedVersionOfSignature:withLocationReset:] (KmodSymbolLookup.m:708)]
It seems likely that Shark was broken by a kernel upgrade. I've filed an Apple bug report #9648225.
This problem was fixed for me by reinstalling XCode from the 10.6.6 DVD.