If the Google says "Adsense is already affiliated with your e-mail account " it means that you have an approved Adsense account. You dont have to apply for Google Adsense !!!
Just type http://google.adsense.com and login with your email id and password. If you are able to login means you have an approved adsense account.
Then it is just a matter of configuring the adsense id in Hubpages and the Hubpages URL in the adsense.