HT1277: Configuring Mail for your email accountLearn about Configuring Mail for your email account
MobileMe was discontinued as of the end of June. Did you transition your account to iCloud? If so, something here may help:
If not, you may want to post in the iCloud forum where you may get additional suggestions.
If you did not migrate your MobileMe account to iCloud before 1 August then your account and all its contents have been deleted.
You can re-activate the @me.com address itself but you will need Lion 10.7.2 or an iOS5 device to do it; just sign in at System Preferences>iCloud or Settings>iCloud, and activate Mail in the checklist there - however you cannot do this on Snow Leopard, which your signature says you have.
If you are able to open an iCloud account; or if you did in fact migrate the account in time, you will need to set Mail up manually in Snow Leopard: