I believe a step or two in the
java library configuration instructions was missed, specifically these one:
1) Add a Java Build Variable for the navX-MXP Java Library. To set the build variable, from the Eclipse menu select Window->Preferences->Java->Build Path->Classpath Variables, then click on the “New…” button. Then, in the “New Variable Entry” dialog, set the name to “navx-mxp” and set the Path to <HomeDirectory>\navx-mxp\java\lib\navx_frc.jar. Hit OK to save this change.
and...
2) Ensure that your project’s library paths include the navx-mxp variable, as well as WPI library path variables (networktables and wpilib). If any are missing, you can add them by clicking on the “Add Variable…” button.
If either the "navx-mxp" build variable is not present in the Eclipse Java ClassPath Variables settings - or if your project's library paths do not include this "navx-mxp" variable, the error message you have reported is the very likely result.