I could use a little more clarification - what do you mean by "Open Resource"? Where, and under what circumstances, do you see this option? I looked through the main menu and various context menus and have not found such an option.
OK, now that you told me where to look I see it. I was in "Java EE" and then tried "Java" and "Resource" perspectives and it was there on all three. But not in "Team Synchronizing".
While in a perspective that doesn't show the desired menu item, go to Window | Customize Perspective. Then in the Menu Visibility tab, expand Navigate and then click on the grey-out Open Resource entry. A dialog box will pop up that tells you which Command Group that menu item belongs to. Then go to the Command Groups Visibility tab, find that command group and check it. Then click Ok on the dialog box. You should then have that menu option.