IP Messenger for Eclipse

ご提供されているpluginをインストールしたのですが、以下のNoClassDefFoundErrorが発生します。
インストール方法が間違っているのでしょうか？

「IPMessengerListener」に「java.lang.NoClassDefFoundError」と出ているのですが、
「jp.eisbahn.eclipse.plugins.ipmsg_1.0.0.jar」内の「ipmsg-core.jar」に入っている事は確認しています。

≪インストール方法≫
SourceForgeよりダウンロードした４つのJarファイルを以下のように配置
eclipse\features
+- jp.eisbahn.eclipse.features.ipmsg.ui_0.0.1.jar
+- jp.eisbahn.eclipse.features.ipmsg_1.0.0.jar
eclipse\plugins
+- jp.eisbahn.eclipse.plugins.ipmsg.ui_0.0.1.jar
+- jp.eisbahn.eclipse.plugins.ipmsg_1.0.0.jar

≪環境≫
・Eclipse SDK：Version: 3.1.0
Build id: I20050627-1435
・JVM：1.4.2_05

≪SessionData≫
eclipse.buildId=I20050627-1435
java.version=1.4.2_05
java.vendor=Sun Microsystems Inc.
BootLoader constants: OS=win32, ARCH=x86, WS=win32, NL=ja_JP
Command-line arguments: -os win32 -ws win32 -arch x86

≪StackTrace≫
!ENTRY org.eclipse.ui 4 0 2006-04-13 00:20:26.843
!MESSAGE jp/eisbahn/eclipse/plugins/ipmsg/IPMessengerListener
!STACK 0
java.lang.NoClassDefFoundError: jp/eisbahn/eclipse/plugins/ipmsg/IPMessengerListener
at java.lang.Class.getDeclaredConstructors0(Native Method)
at java.lang.Class.privateGetDeclaredConstructors(Unknown Source)
at java.lang.Class.getConstructor0(Unknown Source)
at java.lang.Class.newInstance0(Unknown Source)
at java.lang.Class.newInstance(Unknown Source)
at org.eclipse.core.internal.registry.ConfigurationElement.createExecutableExtension(ConfigurationElement.java:162)
at org.eclipse.core.internal.registry.ConfigurationElement.createExecutableExtension(ConfigurationElement.java:142)
at org.eclipse.core.internal.registry.ConfigurationElement.createExecutableExtension(ConfigurationElement.java:129)
at org.eclipse.core.internal.registry.ConfigurationElementHandle.createExecutableExtension(ConfigurationElementHandle.java:48)
at org.eclipse.ui.internal.WorkbenchPlugin$1.run(WorkbenchPlugin.java:240)
at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:69)
at org.eclipse.ui.internal.WorkbenchPlugin.createExtension(WorkbenchPlugin.java:236)
at org.eclipse.ui.internal.registry.ViewDescriptor.createView(ViewDescriptor.java:99)
at org.eclipse.ui.internal.ViewReference.createPartHelper(ViewReference.java:267)
at org.eclipse.ui.internal.ViewReference.createPart(ViewReference.java:180)
at org.eclipse.ui.internal.WorkbenchPartReference.getPart(WorkbenchPartReference.java:552)
at org.eclipse.ui.internal.Perspective.showView(Perspective.java:1655)
at org.eclipse.ui.internal.WorkbenchPage.busyShowView(WorkbenchPage.java:930)
at org.eclipse.ui.internal.WorkbenchPage.access$12(WorkbenchPage.java:913)
at org.eclipse.ui.internal.WorkbenchPage$13.run(WorkbenchPage.java:3148)
at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:69)
at org.eclipse.ui.internal.WorkbenchPage.showView(WorkbenchPage.java:3145)
at org.eclipse.ui.internal.WorkbenchPage.showView(WorkbenchPage.java:3123)
at org.eclipse.ui.handlers.ShowViewHandler.openView(ShowViewHandler.java:146)
at org.eclipse.ui.handlers.ShowViewHandler.openOther(ShowViewHandler.java:102)
at org.eclipse.ui.handlers.ShowViewHandler.execute(ShowViewHandler.java:70)
at org.eclipse.ui.internal.ShowViewMenu$3.run(ShowViewMenu.java:113)
at org.eclipse.jface.action.Action.runWithEvent(Action.java:996)
at org.eclipse.jface.action.ActionContributionItem.handleWidgetSelection(ActionContributionItem.java:538)
at org.eclipse.jface.action.ActionContributionItem.access$2(ActionContributionItem.java:488)
at org.eclipse.jface.action.ActionContributionItem$5.handleEvent(ActionContributionItem.java:400)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:66)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:843)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3080)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:2713)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:1699)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:1663)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:367)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:143)
at org.eclipse.ui.internal.ide.IDEApplication.run(IDEApplication.java:103)
at org.eclipse.core.internal.runtime.PlatformActivator$1.run(PlatformActivator.java:226)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:376)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:163)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.core.launcher.Main.invokeFramework(Main.java:334)
at org.eclipse.core.launcher.Main.basicRun(Main.java:278)
at org.eclipse.core.launcher.Main.run(Main.java:973)
at org.eclipse.core.launcher.Main.main(Main.java:948)