This one took me a few minutes and some digging to figure out. Once I found instructions the install was very easy. Hopefully, this can save someone some time. One more thing. These instructions reference the Mambo plug in; however, it appears that the info also works for J! Good Luck!

These instructions are from Open4G.com

1:) Installing NVU Mambo (J!) Template Builder extension

Open NVU, in menu top locate "Tools" option, next click in "Extensions", this will open NVU extensions manager.
In extensions manage, click on "Install" button and browse and select open4gtemplatebuilder.xpi file and install it on NVU.

Exit from installation and close NVU Application
Now restart NVU.You will see a new Toolbar on NVU application.

Further instructions can be found at this link.

http://www.open4g.com/index.php?option=com_content&view=article&id=15%3Ausing-nvu-mambo-template-builder-extension-in-nvu10pr&catid=13%3Atemplate-builder-tutorials&limitstart=1

Now, I haven't gone through the entire process yet...but it appears that this will work.