[news.eclipse.newcomer] Manifest.mf Problem

[ihybily@xxxxxxxxx (amy)]

Hi, I'm encountering a problem with my plugin in eclipse3.2.1

i'm doing the very first example in the book "contributing to 
Eclipse".---the easist plug-in "org.eclipse.contribution.hello"
after i wrote these in the plugin.xml
    <plugin>
	<extension 
		point="org.eclipse.ui.actionSets">
	  <actionSet
   	   id="org.eclipse.contribution.hello.actionSet"
   	   label="Hello Action Set">
   		<action
     		 id="org.eclipse.contribution.hello.HelloAction"
     		 label="Hello">
		</action>
	  </actionSet>
	</extension>
     </plugin>

i got an error in the MANIFEST.MF
"Plug-ins declaring extensions or extension points must set singleton 
attribute to true"

so I added the 
singleton:=true 
to my 
Bundle-SymbolicName: org.eclipse.contribution.hello
-->
Bundle-SymbolicName: org.eclipse.contribution.hello; singleton:=true

I still got the original error.  :(

can anyone help?
thanks in advance
amy