Apple has issued a statement stating that the near defunct MobileMe service which has basically been replaced with iCloud would be killed off for good on June 30th. Mac users running an OS X version older than Snow Leopard cannot use iCloud because it is not compatible. The only way for users to gain access to iCloud is to have Snow Leopard installed after which you must install OS X Lion from the Mac App Store and then you will have access to iCloud. If you are still running MobileMe, the safe money would be on the fact that you are probably not running the latest version of OS X.
A report from Macgasm indicated that Apple has started emailing remaining MobileMe users in a bid to get them to install Snow Leopard from an installation DVD they can now get for free. Although the OS was released way back in 2009, Apple is still listing it for $30 on its website. MobileMe users now have the opportunity until June 15th to grab the software for free via 2 methods. The first is to follow the directions from Apple in the email that was sent to you, the second is to follow this link, login with your MobileMe account and claim your copy of Snow Leopard.
If you want to gain access to iCloud, you will need OS X Lion and it costs $30. Since the only place you can get is from the Mac App Store, after installing Snow Leopard, head on over there and complete your transition from a fading out MobileMe to the awesome iCloud that other Apple users have been enjoying for the past 7 months.