Ask for serial number http://forums.adobe.com/thread/1234635 has a FAQ link
Log out of your Cloud account... Restart your computer... Log in to your paid Cloud account
You can also check if the host file has Adobe entry or not as if Adobe entries are there then also CC can ask for serial number. You can check the host by the followinh method.
Creative cloud do not need a serial number. it will be using your Adobe ID on which you have purchased the creative cloud membership.
So you need to login with your Adobe ID and password to activate the cloud membership.
Log out & log back in of the CC Desktop App.
In case it is not signing in successfully please try the following:
I don't know which operating system you are working on so i am giving you some steps windows and MAC OS:
In windows 7 navigate to following location:
1. look out for "Hosts" file
2. Open it with notepad
3. Check if you have any entry for Adobe
4. Remove the entries and try again launching any product from CC
1. Please click on "Go" and navigate to /private/etc
2. Open "hosts" file and check out for any entries for Adobe.com
3. Remove the entries and save the file
4. try again launching any product from CC