The key here is getting Eclipse to find out about the various classes that the XPages runtime uses, which are stored in OSGi plugins, which need to be added to Eclipse's active Target Platform.
There are a few ways you can go about it. Particularly if you're running on Windows, the best route is likely the XPages SDK:
http://www.openntf.org/internal/home.nsf/project.xsp?name=XPages%20SDK%20for%20Eclipse%20RCP
That will guide you through pointing Eclipse to your Notes/Domino installations and setting up the plugins appropriately.
Alternatively, the "manual" route is to add these plugins to the Target Platform yourself. The "cleanest" path to that is the Update Site for Build Management:
http://www.openntf.org/main.nsf/project.xsp?r=project/IBM%20Domino%20Update%20Site%20for%20Build%20Management
You can download that, extract it somewhere, and then go to Eclipse's preferences → Plug-in Development → Target Platform, either add a new one or Edit the existing, and then add a new "Folder" source that points to the root of that extracted Zip (some other source types may also work).
If you want to use IBM's Extension Library (the one on extlib.openntf.org), you could do a similar thing: download it, extract it, and then point to it in the Target Platform.
You could also point these Target Platform directories to those used in Notes or Domino (this is what the XPages SDK does), in which case I believe the folders you'd want are:
- (install dir)/framework/rcp/eclipse
- (install dir)/framework/shared/eclipse
- (data dir)/workspace/applications/eclipse
That would cover the base XPages plugins, a number of likely-unnecessary Notes/Designer-specific plugins, and any custom plugins you have installed.