This the correct URL:
http://trailers.apple.com/
Do you not see them there?
The following only applies to those running Tiger or Leopard:
If you are experiencing trouble viewing QuickTime movies with QuickTime Player or the QuickTime browser-plug-in follow the steps in the link below to ensure that QuickTime is properly installed on your Mac
http://www.apple.com/quicktime/troubleshooting/
If you having problems watching WWDC Keynotes, tutorials or Trailers on the Apple website, check your [b]Quicktime Player 7[/b] settings in System Preferences:
Under Streaming, set for Automatic, enable Instant On and Play Immediately.
Under Advanced, click Mime Settings and ensure that Streaming (both kinds) are enabled.
(Note that if you are running Snow Leopard you should be using Quicktime Player 7 and not Quicktime 10.) Snow Leopard does not have a QuickTime System Preference pane. No MIME Settings options. No streaming settings. If a User has migrated from 10.5.8 then those settings have moved over (but no changes can be made). QuickTime Player 7 is in your Applications/Utilities folder (if you've got it installed). If not you can install it from the Snow Leopard DVD (Optional Installs).