did you copied URL: http://beust.com/eclipse in below path
[attachment=3:miqb3i16]Testng-Installnewsoftware.png[/attachment:miqb3i16]
[attachment=2:miqb3i16]bestuse.png[/attachment:miqb3i16]
After adding URL "http://beust.com/eclipse "in "Work with", click on "Finish" button/Enter
you can cross check whether Testng is installed or not by navigating to Window -->Show view --> Other --> Testng
[attachment=1:miqb3i16]Testng-plugin.png[/attachment:miqb3i16]
[attachment=0:miqb3i16]plugin.png[/attachment:miqb3i16]