Quick tutorial:
In eclipise:
-import project from cvs, host: dev.eclipse.org, repository path:/cvsroot/webtools, user:anonymous, connection type:pserver
-click next, find and choose plugin org.eclipse.jst.pagedesigner and import it to your workspace
-create Hello RCP
-in plugin.xml tab extension add new: org.eclipse.ui.editor
-in this extension point add new element, editor and set: id- no matter what, but remember it for future (e.g. "editorId"), extensions: xml,html,xhtml class:org.eclipse.jst.pagedesigner.editors.HTMLEditor, contributorClass: org.eclipse.jst.pagedesigner.editors.actions.PageDesignerActionBarContributor2, default: false.
-Now you have to open a file, "transform" to IEditorInput
-call method IWorkbenchPage.openEditor(iEditorInput,"editorId")
Should work!