I never used virtuNewsletter myself, so take this answer with a grain of salt.
“Subject” seems to be the subject of the newsletter-email. The content of the newsletter-email is defined in the resource you set with “Resource’s ID”.
I think you are mixing things up here:
These 3 resources are needed to handle subscriptions, confirmations and reading the newsletter via web.
For the email texts you can either create entries in the CMP under “Templates” or alternatively create new resources. If you use resources, you have to set the system settings virtunewsletter.subscribe_confirmation_tpl, virtunewsletter.subscribe_succeeded_tpl, etc. so the program knows where to read the email texts from.
I don’t think “activation-email.tpl” is used in the code. You probably have to copy the content to “Templates” in the CMP or into a new resource.
In general, the documentation doesn’t seem to be up to date. I don’t know how well maintained this extra is, but maybe you want to look at alternatives.