You appear to be using a third party site for another service to then add Nabble's embedding code to your blog.
The key lines in the instructions you have found are:
===============
<b:if cond='data:blog.url == "FORUM URL"'>
CODE FROM NABBLE
</b:if>
===============
For support with anything else you really need to turn to the two sites with which you are already working. Nabble won't know the detail of those sites or how good their instructions are.
We might be able to give better help here if you actually posted the URL of your Nabble forum and the embedding code you find under:
Options > Embedding options
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.