Like most products on OS X, NI software does not have an uninstaller. This is primarily because the installation process is relatively simple, with very few files being installed in common places. There are exceptions to this, but generally for software it remains true.
Hardware, on the other hand, does usually place extensions (files) in the system folder. For hardware, an uninstaller may be used to avoid manually deleting system extensions.

Native Instruments installers write files into the following directories (the ~ symbol means your user home directory)
MacintoshHD/Applications (unless otherwise specified at installation time)
MacintoshHD/Library/Preferences
MacintoshHD/Users/~/Library/Preferences
MacintoshHD/Library/Audio/Plug-ins/ VST
MacintoshHD/Library/Audio/Plug-ins/Components
MacintoshHD/Library/Application Support/Digidesign/Plug-ins


For sample line products using DFD, the following directory is also created:
MacintoshHD/Library/Application Support/Native Instruments

Sherlock
The easiest way to remove a Native Instruments product from your system is to use the Finder (or Spotlight). Search for the product name, then delete the returned results. For example, to safely remove Kontakt 2 from your computer:
Press Apple-F in the Finder to bring up Sherlock
Set the filter for "Name", "contains" to return the best results
Type Kontakt 2

Search Results
In the upper pane, select the file or folder according to the location (as noted above)
Select the folder or file in the lower pane of the results window
Press command-backspace to delete the file


Alternatively, you may navigate to the above directories and manually delete the files / folders. As a rule of thumb, the files associated with the product usually contain the product name. For example, the Kontakt 2 preferences file is called Kontakt 2.plist.