I got wonder when I try to find place to add External Extension in Eclipse 3.4 because it doesn’t have the URL as: Help -> Software Updates -> Manage Configurations as Eclipse 3.2.x

On the net, I found the solution, here it is:
Go to: Help -> Software Updates : One windows will come up
On Tab Available Softwares, Click on Add Site and Then Choose Locale Button to browse for the plug-in to add
Finally Click Install

See the screen shot: