First verify if it was installed correctly by checking the installed plugins
Help -> About Eclipse -> Installation Details -> Plug-ins
If it is, the question is whether you use a Target Definition to which you need to add the Software Site.
If it is not, you need to check why it wasn't installed. I can't say anything about that with the given information.
You should be able to simply add NatTable to the Plug-in dependencies of your plugin project.