Hi,
there is a newsletter subscription web part and you can insert it wherever you need:
Integreting newsletters to the siteAs for combining registration and subscription functionality. You will need to
modify code of a standard web part.
How to add a subscription using API is shown in your 6.0 Kentico installation, on the page
http://your-domain/CMSAPIExamples/Default.aspx, in the section On-line marketing - Newsletters - Subscribers.
Best regards,
Ivana Tomanickova