It is possible that you haven't been approved yet (it can take several weeks depending on your country).
If you have been approved, simply login to AdSense here...
and verify that you can login.
Once you can, simply go to https://hubpages.com/my/affiliate and start the process to link your HubPages account with your AdSense account. You will be asked for your zip code, AdSense email address, and the last 5 digits of your phone number. The screen looks like this:
You will then need to go back to AdSense and approve HubPages access to your account. You should get an email with directions on how to do that.