How are you trying to install it...?
From your BB or from your PC?....if you are using your PC and using FireFox that explains the issue..try it in IE ( wow hurts bad to tell someone to use IE :P )
If on your BB you should be able to go to http://mobile.blackberry.com
and choose on the home screen where it says "Instant Messaging For BlackBerry SmartPhones"...when doing it from your BB though, make sure your emulation mode for your browser is set to "BlackBerry" otherwise it will toss out the same error the PC does. (to change emulation mode Open browser, Menu Key, Options, Browser Configuration..then look for "Emulation Mode")
Hope this helps ya out