Again, when I say to use "Open in Timeline" that means to select the clip in the event browser. The thing in the upper left hand corner. Then choose from the Clip menu the option "Open In Timeline." This will then let you make changes to the clip as it exists solely in the Event Browser. It will show you the clip's video/audio inside a timeline, but the timeline is not for any project. As Tom describes, it is the "clip timeline."
To actually do the resync, you will need to detach the audio from the video. Then you can move the audio (at the sub-frame level by zooming in) to sync it.
FYI, you can actually do this for any clip, even ones in a project timeline. Changes you make in a project timeline however will not affect the "master" clip it came from in the event browser, or any other instance of that clip. So that's why, in this case, I'm telling you to make the sync change in the Event, so that whenever you do use that clip for a project, the changes will also be used.
Finally, if you are talking about syncing audio and video where the audio is from a separate source, that's a slightly different process. If that is the case, you need to select both the audio and video in the Event Browser, and choose "Synchronize Clips" from the Clip menu. This will create a compound clip inside the Event browser with the application's best attempt at doing an automatic sync. You can manually (re)adjust the sync by following the instructions above -- open the clip timeline and make your changes.
Hope this helps. I know the app can be confusing, especially coming from, well, anything but iMovie I suppose. Try to hang in there though.