Take a look at GeoSteiner v5.1 which is available for free download at http://www.geosteiner.com. It is generally considered to be the best solver of these problems available. I have been working on trying to compile it on my Mac so that I could then call it from within Mathematica. Unfortunately, I keep getting a couple of compiler errors which seem to be related to the Mac's standard implementation of libtools.
I myself would be very interested to know if anyone has successfully complied GeoSteiner on the Mac and, if so, how did they do it? It would be great to have this powerful software accessible within Mathematica.
With some very kind help from Szabolcs I was able to get GeoSteiner working on my Mac. In the hope it may help someone else here is how I did it:
To install GeoSteiner on a new system proceed as follows:
Install Xcode from the Mac App Store.
Install Command Line Tools by executing the following from Terminal:
Accept the Xcode license by executing the following from Terminal:
Download the MacPorts package file that applies to your version of OS X and install it by double-clicking on the package.
Install libtool from MacPorts by executing the following from Terminal:
sudo port install libtool
Quit and restart Terminal. Execute the following from Terminal to verify that GeoSteiner works:
To periodically update MacPorts execute the following from Terminal:
sudo port selfupdate
port upgrade outdated