1. I am using the default theme as well.
2. That I have re-installed it just in case there was a problem
3. That I have a fair understanding of PHP, and have coded for many years…..
4. I cant get it working at all – the <!–nextpage–> just gets printed to the page – just like it has done here!
My guess is you are inserting it into the wysiwyg animal… if you see it printed out.
Turn off the wysiwyg (I never use it, that might be the explanation) and it should work. To turn it off: Users > Your profile.