You need an Adsense account to activate it. Some people who join HP without an Adsense account are able to apply for one using their HP account as their "website" although this is getting harder to do. The easiest and fastest way to get an Adsense account is to apply with a website that you actually own. That could even be something as simple as a Blogger blog. You can also apply with a YouTube channel. You need to have ~20 blog posts or videos before they consider your application, but once you are approved, you can insert your Adsense ID number in your HP account settings and start earning here. Blogger and Youtube are both owned by Google, so that's why they are easier routes to getting Adsense.
You can help the HubPages community highlight top quality content by ranking this answer up or down.