Well here is a quick rundown (I don't have my Mac here at work - so this from memory):
1) Install the Apple Developer Tools
2) Download and Install Mac Ports
3) Install the following packages using ports (these are from memory so the list may be incomplete and/or inaccurate):
libsdl
libsdl_net
libpng
libsdl_sound
Hint: On the Command Line, to install a package with ports, use the "port install <packagename>" syntax.
4) Download the latest DOSBox cvs code from a prepackaged tarball here or use the CVS download instructions here.
5) Extract the dosbox source if you downloaded the tarball. The source will be extracted into a subdirectory called dosbox. If you pulled the latest code from CVS, the source will be in a subdirectory called dosbox as well.
6) Enter the dosbox subdirectory in the terminal.
7) Type "./autogen.sh"
8) Type "./configure"
9) Type "make"
Hint: If any of the above three commands fail, then you either a) are missing an important package, b) didn't install the dev tools or c) got a version of the CVS code that has a compilation problem. For example, in older versions DOSBox wouldn't compile right on OS X if you had SDL_net installed due to a bug that has been fixed in the most recent CVS snapshots.
10) Enter the "src" subdirectory.
11) Execute the new build by typing "./dosbox"
Note: You can also update the DOSBox 0.70 Application Bundle with the new binary. Simply replace the DOSBox.app/Contents/MacOS/DOSBox file with the new dosbox binary you execute in step 11. Please use the exact case that the old executable had (i.e. the resulting binary in the bundle should be called DOSBox rather than dosbox).
Hopefully that will help. I probably got the package names for MacPorts wrong. You can run "port search sdl" on the commandline for a list of the most of the packages above. That will give you the proper package names for the "port install <packagename>" command.
Good luck!