I can't sign into my account because I do not know the slug.
A slug is the 'name' of your NationBuilder site. So for example, my site is nbdemo.nationbuilder.com - with nbdemo being my slug!
If you have a custom domain you can get to your slug by going to yoursite.com/admin and it will redirect to yourslug.nationbuilder.com/admin so you can see it!
Complete and current information on how to use NationBuilder is available in the documentation section.