How to build Primecoin from source code on a mac? How should one build the client for the new cryptocoin Primecoin from source code on a mac? 

The source can be found here: https://sourceforge.net/projects/primecoin/files/
https://github.com/bitcoin/bitcoin/blob/master/doc/build-osx.md should work for you as Joe White commented, note you'll have to install Xcode first of all, and in Xcode open preferences - downloads and install the command line tools. I recommend homebrew over macports as it's better maintained and a little more user friendly.

the only difference is that as you're downloading from sourceforge rather than github, you skip the git clone and just find makefile.osx in /src/src/ of the downloaded folder.

you then (assuming you installed homebrew as standard) open makefile.osx in textedit or xcode and change

DEPSDIR=/opt/local


to

DEPSDIR=/usr/local


and you'll probably have to change

 $(DEPSDIR)/lib/db48/libdb_cxx-4.8.a \
 $(DEPSDIR)/lib/libboost_system-mt.a \
 $(DEPSDIR)/lib/libboost_filesystem-mt.a \
 $(DEPSDIR)/lib/libboost_program_options-mt.a \
 $(DEPSDIR)/lib/libboost_thread-mt.a \
 $(DEPSDIR)/lib/libboost_chrono-mt.a \
 $(DEPSDIR)/lib/libssl.a \
 $(DEPSDIR)/lib/libcrypto.a \


to paths more like

 $(DEPSDIR)/Cellar/openssl/1.0.1e/lib/libssl.a \
 $(DEPSDIR)/Cellar/openssl/1.0.1e/lib/libcrypto.a \


Check in /usr/local for the Cellar folder and the libs folder, you could alternately alias each referenced file in /usr/local/libs