Did you restart Eclipse after installing the plugins? After you install them, you have to restart Eclipse for them to work correctly. If you did, then there are a few things you can check.
First, make sure that Eclipse on all the machines is actually showing the "Referenced Libraries". See
this image to see how to check.
Secondly, check the "Robot Java Development" plugin version on all platforms via the Eclipse "Installation Details". If you don't see the plugin in the list, you didn't install it.
Thirdly, check your home directory ("/Users/<username>" on Mac and Linux, and IIRC "C:\Users\<username>" on Windows) to see if it contains a "wpilib" directory. This directory is populated the first time you create a new robot project using the plugins, so if you don't see it verify that you tried creating a new robot project.
Finally, check one of your robot projects to see whether the build path includes the WPILib libraries. Do this by selecting the project, then going to "Project" menu and selecting "Properties", which will open up a dialog window. Select "Java build path" in the left side of that window, and then the "Libraries" tab in the right side of the window. You should see entries for "networktables" and "wpilib" with paths to where those libraries are installed.
If none of these work, then try uninstalling the Eclipse plugins, restarting Eclipse, reinstalling the plugins (carefully checking the update site URL), restarting Eclipse, and then creating a new robot project to see if that helped. If not, then we'd need to know the results of each of the above checks, the
exact update site URL, plus the
exact versions of: the operating system, JDK (including vendor), Eclipse, and the "Robot Java Development" plugin.