Basically, at the time of writing you can’t actually do this with Safari 5.1. However you can still use Safari 5.0.5, which does support the plugin. This thread has a detailed explanation of the problem, so I won’t go into it. You can read through the thread to find the solution, or follow the below steps:

• Install Pacifist
• Download Safari 5.0.5
• Use Pacifist to extract the Contents/Safari.app file someplace, such as your Desktop.
• I then renamed Safari.app to Safari 5.0.5.app, modified the Contents/Info.plist to make the running name match (Change line 463 for the key CFBundleName to a value of your liking), copied the app package into /Applications, and further wrote over Contents/compass.icns with the following file to make the icon look retro (so I know at a glance its the old version). I put the file up so others can use it too. Retro Safari.icns