Do I access it through Invision, the domain or do I need an html editor application?

Thanks.

It depends on which ad code you're looking to integrate.  If you're doing autos, you need to add it to the header section of your theme.  

ACP > Customization > Appearance > Themes

If you're placing individual ads, you would place it using the advertisement system.

ACP > System > Site Promotion > Advertisement

The above will show you how you can add individual ads. If you are looking to add something more complex, you would be using the template system. It would however be unusual to ad adverts in this manner