Pkg-config is a helper tool used when compiling applications and libraries.  If you want to build Mono apps from source using configuration scripts, you will need to put the Mono.pc path in your PKG_CONFIG_PATH environment variable.  If it’s not set, you will get an error like “configure: error: missing the mono.pc file, usually found in the mono-devel package” or “Failed to initialize the ‘Mono 3.5 Profile’ (mono-3.5) target framework”

Here’s how to customize your terminal prompt.   To add the location of mono.pc, edit .profile or .bashrc in the root of your home folder, and add this line

export PKG_CONFIG_PATH=/Library/Frameworks/Mono.framework/Versions/Current/lib/pkgconfig/"

Here is my full .profile file:

export PATH=/Applications/Windows/Darwine/Wine.bundle/Contents/bin:/opt/local/bin:/opt/local/sbin:$PATH
export PKG_CONFIG_PATH=/Library/Frameworks/Mono.framework/Versions/Current/lib/pkgconfig/
export DISPLAY=:0.0

(When using MacPorts, the path is /opt/local/lib/pkgconfig/)