This is all very bizarre. When you migrated from MobileMe you should have been able to sign into System Preferences (or Settings)>iCloud with the ex-MobileMe address; on enabling Mail you then have that address to use, as well as it being the ID. When signed in, and checking 'Mail' in the checklist, it should simple set your Mail application up with the details of the email account, which once your had migrated was already active (you would have been able to set the email up manually on a pre-Lion computer, for example, which is exactly what I did). So you wouldn't be enabling email, simply automatically configuring the Mail app.
Of course if you then create a new iCloud account you can't move the ex-MobileMe address to it as that is an ID it its own right, so you would effectively be merging accounts, which isn't possible.
You may have already been walked through this by Apple, but I would suggest you try this:
Firstly, sign out of iCloud on all your devices. Your data will be removed from them but not from the server.
As a check, go to http://icloud.com , sign in with your ex-MobileMe address, and confirm that you can access your email there.
Now on your Mac go to System Preferences>iCloud and sign in with that address. Check Mail in the list, Go to the Mail application and confirm that you have an iCloud account there and that the settings are correct. If there is mail on the server it should be visible.
Now on your phone, go to Settings>iCloud and, again, sign in with the ex-MobileMe address and enable Mail in the list. Check the Mail app.
If you go through all this and it doesn't work, at what point does it fall over, and what sort of error message do you get?