That file you are trying to download is only for Firefox 3.5.
For IE when you play a game the first time
You will see a yellow bar at top of the window
Click Install This Add-on from the yellow bar
A box will appear click Install
Restart Internet Explorer
Play game
For FF 3.6 which is what you have see this thread
http://retrouprising.com/support/ret...8-a/t8806/new/