To the Webmasters/ Editors

Respected Webmasters/ Editors
When I tried to apply for the Google Ad Sense Account I get the message that "It appears that you already have a Google Ad Sense Account..." guide me as I don't have such an account anywhere. Is this problem because I am having an Email ID with gmail(Google) itself. Should I change my Email ID. Also is I am in little bit doubt for digital IP signature. What is that? What should I write in that column? Should I write the IP address (in numeric) written next to the box? Guide me as I want to apply for the same.