There is a lot of Eclipse non-sense out there that you have to wade through. The best book that I have found on developing plugins is Eclipse Plug-ins, Third Edition by Eric Clayberg and Dan Rubel.
How do I include an external (third-party) JAR file in a plugin?
First start is to head off to the Eclipse Plug-in Development FAQ. where you'll find that you should not use the project's Java Build Path. This is a red herring.
The correct answer is here, buried in the FAQ. Ignore any other resources that you may come across; they are wrong.
Third-Party Library Nonsense
Access restriction: The type WordNotFoundException is not accessible due to restriction on required library: <snip>. One solution is here.
Eclipse uses Bundle resources, which not all APIs can handle. If you need the physical path to a resource you can use FileLocator.resolve().