i downloaded a file with the quicktime icon next to it. when i tried to view it i got an alert message that says,

Alert, QuickTime cannot open the file: "br*k*b*ck_m**nti*n_full_movie_MPG" is not a file QuickTime understands (-2048)

Please don't mention file names you're downldling if they are copyright! Forum Rules (click here)

That means you're lacking the plug-ins to play the file. Best for p2p previewing & playback is VLC - (click on link) _ (choose nearest mirror site) as it has in-built codecs, etc. & is very reliable.

after i download this, then do i launch the movie or do i have to do something else?

You'll need to make VLC the default for that type of file if you wish to launch it via LW. Otherwise you can open VLC & direct it to open the file from your downld folder. To make a program default, select the mpg file, use command i (⌘ i) & select Open with ... & choose VLC on the list & then press the option for Change all.