If you're having troubles opening the jar file...

[Diet6]

(if you need just get the installer running, try dropping the file you download on the javaws.exe found in your java directory)
Once you have it installed and have a .jar file, if you can't open it, try this:
Create a shortcut with the link:

"C:\Program Files\Java\jre[[your version number]]\bin\javaw.exe" -jar "C:\Program Files\MMOUI Minion\minion.jar" %

Remember to replace the directories with the ones you have as they might be different. This shortcut should ignore any previous file associations with other applications.

Enjoy