.Mac or Mobile Me accounts cannot be directly used to sign in to MSN Messenger since this in on a different IM service. You need to have a Windows Live account for MSN...this however does include the ability to create a Windows Live account, without creating a Hotmail account using your current email address.
Go here:
https://accountservices.passport.net/ppnetworkhome.srf?vv=550&lc=1033
And choose the "Use an e-mail address you already have" option.
-Kevin