When I first tried to integrate my existing TNG site with a WordPress blog, I had difficulties getting the integration to work. I first tried the instructions at http://tng.lythgoes.net/wiki/index.php?title=Using_TNG_and_WordPress_with_the_tng-wordpress-plugin. I had so much trouble with these instructions, that I ended up having to uninstall WordPress completely. The process I eventually used was found at http://www.kloosterman.be/info/how-i-made-my-tng-wp-website/
Once the WordPress/TNG integration is working, the next thing to do is to make sure that the WordPress Theme you are using works properly with the TNG pages. I had a lot of trouble when I first integrated TNG and WordPress trying to find a WordPress Theme that would work properly with TNG, and that would also allow me to add customised pages. I found the Weaver II WordPress theme the best one to use, as it works well with TNG, and Weaver II has a number of sub themes, so you can find a suitable sub theme if you decide to change your TNG template. There are also a full range of options of modifying virtually any part of WordPress with Weaver II – with the other themes I tried, you were stuck with the one look.