The Visual Page Editor does not work under the final version of Eclipse 3.5 JEE Cocoa for Mac OSX and JBoss Tools 3.1.0.M1, the error is due to a missing cocoa version of XUL runner as you can see in the following stacktrace:

org.jboss.tools.vpe.xulrunner.XulRunnerException: Bundle org.mozilla.xulrunner.cocoa.macosx is not found.
at org.jboss.tools.vpe.xulrunner.browser.XulRunnerBrowser.getXulRunnerPath(XulRunnerBrowser.java:219)
at org.jboss.tools.vpe.xulrunner.browser.XulRunnerBrowser.initXulRunner(XulRunnerBrowser.java:114)
at org.jboss.tools.vpe.xulrunner.browser.XulRunnerBrowser.<init>(XulRunnerBrowser.java:85)
at org.jboss.tools.vpe.xulrunner.editor.XulRunnerEditor.<init>(XulRunnerEditor.java:135)
at org.jboss.tools.vpe.editor.mozilla.MozillaPreview$1.<init>(MozillaPreview.java:59)
at org.jboss.tools.vpe.editor.mozilla.MozillaPreview.createPartControl(MozillaPreview.java:59)
at org.jboss.tools.vpe.editor.VpeEditorPart.createPreviewBrowser(VpeEditorPart.java:921)
at org.jboss.tools.jst.jsp.jspeditor.JSPMultiPageEditor.pageChange(JSPMultiPageEditor.java:248)
at org.jboss.tools.jst.jsp.jspeditor.JSPMultiPageEditorPart$2.widgetSelected(JSPMultiPageEditorPart.java:106)
at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:228)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Display.sendEvent(Display.java:3542)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1247)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1270)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1255)
at org.eclipse.swt.widgets.Widget.notifyListeners(Widget.java:1076)
at org.eclipse.swt.custom.CTabFolder.setSelection(CTabFolder.java:3256)
at org.eclipse.swt.custom.CTabFolder.onMouse(CTabFolder.java:2045)
at org.eclipse.swt.custom.CTabFolder$1.handleEvent(CTabFolder.java:323)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Display.sendEvent(Display.java:3542)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1247)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1270)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1255)
at org.eclipse.swt.widgets.Widget.notifyListeners(Widget.java:1076)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3440)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3099)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2405)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2369)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2221)
at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:500)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:493)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:113)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:194)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:368)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:559)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:514)
at org.eclipse.equinox.launcher.Main.run(Main.java:1311)