Enable new content types to use as newsletter I am using the simplenews module in order to create newsletters and enable users to subscribe to those. By following the instructions in the readme file it says:

Enable new content types to use as newsletter:
Structure > edit content type > Publishing options

What I did, I created a new content type and in the Publishing options I enabled the "Use as a simplenews newsletter" option.

Is this correct? Or should I have gone into one of the existing content types and selected this checkbox instead?

Also, 
ENABLE SIMPLENEWS BLOCK

With the Simplenews block users can subscribe to a newsletter.

Enable a Simplenews block per Newsletter category:
Structure > Newsletters > edit newsletter category

In my block structure I have the following available to me:
Newsletter: Multi Subscription
Newsletter: Newsletter

Enabling Newsletter: Newsletter displays nothing. SHould it?
Enablinb Newsletter: Multi Subscription displays an option of subscribing to available newsletters. But I only have one newsletter so it's a bit misleading.

What settings have I stuffed up in order for the Newsletter: newsletter block to not display anything, but to instead have a multi subscription block?
The main options are the defaults.  So when you edit a content type all the options you turn on/off are the defaults for newly created content.  Any content that has already been created will need to be edited and re-saved with that option selected.  If you have a lot, I suggest using VBO.