Oracle (ex Sun) or Apple Java™[1] runtime 1.6 or above.
At least 1Gb of memory and a 1GHz CPU.
100Mb of free disk space; 200Mb for the self-contained distribution which includes a private Java™ runtime.
XXE is officially supported on Windows XP/Vista/7, on Linux and on Mac OS X 10.5 (Leopard) /10.6 (Snow Leopard)/10.7 (Lion). It is possible to use it on other Java™ 1.6+ platforms, but without support from XMLmind.
[1] Oracle and Java are registered trademarks of Oracle and/or its affiliates. Other names may be trademarks of their respective owners.