How to enable “Edit Page” in SharePoint’s standard form.aspx

You can do it much easier way by tweaking URL. Just make it look like,
newForm.aspx?ToolPaneView=2&PageView=Personal
and you will be in Edit Mode for the page. No need to go to Designer.

catch here for more url tweaks
http://abstractspaces.wordpress.com/2008/05/05/trick-the-sharepoint-query-string-url/