If you set up iCloud using an Apple ID that doesn’t end with @icloud.com, @me.com, or @mac.com, you must set up an @icloud.com email address before you can use iCloud Mail.
If you have an email address that ends with @mac.com or @me.com, you already have an equivalent address that’s the same except it ends with @icloud.com. If you use email aliases that end with @mac.com or @me.com, you also have equivalent addresses that end with @icloud.com.
On an iOS device, go to Settings > iCloud, turn on Mail, then follow the onscreen instructions.
On a Mac, choose Apple menu > System Preferences, click iCloud, select Mail, then follow the onscreen instructions.
After you set up your @icloud.com email address, you can use it to sign in to iCloud. You can also sign in using the Apple ID you used to create your iCloud account.
If you still can not set up Email id, Let me take you through step by step process :
- Go to the Apple menu and choose “System Preferences”
- Select “iCloud” (or “Internet Accounts”)
- Check the box next to “Mail” so that it’s enabled (this will always be unchecked if you don’t yet have an iCloud.com email address)
- Choose the Email id you like to create and click “OK”
Please Note this is one time and permanent, you can’t change the Email ID once created , it binds to the Apple ID, so choose wisely.
- Confirm by clicking Create.
- Launch Mail app in Mac OS X and the new Email ID “firstname.lastname@example.org” is created and added to the Mail app – you’ll get a welcome email confirming Email ID.
Source : Apple Forums