Sorry, as Tom says we clearly made a mistake and didn't set up the link at the website properly. I'll try to get that fixed.
Another option is to build from source. I don't know if you want to go down that route.
If you are building from source you can get Open MPI from here:
http://www.open-mpi.org. Recent versions should work.
We now build Xyce using CMake and preliminary instructions for this are in the file "INSTALL.md" in the top level of the Xyce source tree. The Xyce source tree can be gotten from github here:
https://github.com/Xyce/Xyce
thanks,
Eric