How can I Include an installed module in customer RCP application?

Thanks Tim, we are getting closer.  I am on a PC running Windows 7 64 bit and I found that the pluggins I download and install get placed in the c:\users\<user>\AppData\Roaming\NetBeans\8.1 directory rather than in the installation directory (c:\Program Files\NetBeans 8.1.

I assume that by default NetBeans only shows clusters in the Libraries window that are in the installation directory and since the installed pluggins are not placed there they are not available for me to include in my application.

Based upon your comment, I noticed the following in the netbeans.conf file:

# Additional module clusters:
# using ${path.separator} (';' on Windows or ':' on Unix):
#
#netbeans_extraclusters="/absolute/path/to/cluster1:/absolute/path/to/cluster2"

I assume I have to uncomment the line in the conf file and put the path to each cluster that I want NetBeans to include in the Libraries window? 

What about the netbeans.clusters and import files?  What are they used for?