I got this error after installing the Mono framework on MacOS X 10.5.6 and trying to set up a new project with eclipse.

NAnt 0.86 (Build 0.86.2898.0; beta1; 12/08/2007)
Copyright (C) 2001-2007 Gerry Shaw
http://nant.sourceforge.net


For more information regarding the cause of the build failure, run the build again in debug mode.

Try 'nant -help' for more information

BUILD FAILED

Failed to initialize the 'Mono 3.5 Profile' (mono-3.5) target framework.

    Function call failed.
Expression: ${not pkg-config::exists('mono')}
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^

        'pkg-config' failed to start.

            ApplicationName='pkg-config', CommandLine='--exists mono', CurrentDirectory='/Library/Frameworks/Mono.framework/Versions/2.4/share/NAnt/bin/'

A few google searches suggested that this might be because pkg-config wasn’t installed - but it’s included in the Mono distribution. I finally realized the symlink wasn’t being created in by the Mono installer. Running the below command fixed everything.

sudo ln -s /Library/Frameworks/Mono.framework/Commands/pkg-config /usr/bin/pkg-config