Hello Yoakshay,
Thanks for using Apple Support Communities.
From your post I understand that you want to sign up for iCloud with your @mac.com Apple ID. See below on how you can accomplish this process:
I had a MobileMe account. How do I sign up for iCloud?
To use your MobileMe account for iCloud, follow the iCloud setup instructions and enter your @me.com or @mac.com email address as your Apple ID. If you had an @me.com address, it will be reactivated for email when you sign up for iCloud. If you had an @mac.com address, it won't be reactivated for email, but you can set up a new @icloud.com email address to use with iCloud.
Important: You will need a verified email address associated with your @me.com or @mac.com Apple ID to set up iCloud. To add a verified email address, go to My Apple ID (appleid.apple.com), sign in with your @me.com or @mac.com email address, and add a valid email address to your account.
Creating an iCloud account: Frequently Asked Questions
http://support.apple.com/kb/HT4436
Take care,
Alex H.