To run Tilemaster, unzip the tilemaster.zip and execute the start.bat script if you are using Windows, or start.sh if you are using Unix (maybe works for MacOS X, too). If the start script doesn't work, you can try to double click the tilemaster.jar file. If you have java 6 properly installed this should start Tilemaster, but it doesn't load the plugins (which at the moment only seems to work with the start.bat/start.sh scripts).
If you are familiar with the command line interface of your OS, this line should start Tilemaster and also load the plugins:
java -classpath plugins;Tilemaster.jar;lib\Itemizer.jar tilemaster.editor.TilesetEditor
For more information please check the wiki:
The tilemaster-trunk.tar.gz archive contains the sources, plugin sources and example data for this release.