Note that MobileMe trial requires a credit card number (presumably to allow annual fee to be automatically charged at the end of the 60 day trial).
There is no way to delete the credit card number to prevent the automatic charging at the end of the 60 day trial (in case you miss the deadline with your activation code).
Entering your activation code prior to the end of the 60 day trial means you lose the rest of the 'free time'.
So, ideally (i.e. to get the full 60 day free trial), you'll need to pay the first year's fee on credit card, and then use activation codes for your 2nd year onwards.
From MobileMe help:
Using an activation key to create, upgrade, or renew a subscription
You can create a new account, upgrade an existing account, reactivate an expired account, or renew an existing MobileMe subscription with an activation key, a unique alphanumeric code that gives you credit toward the purchase of MobileMe services.
NOTE: If you choose to upgrade an existing account or renew an expired account, you will need the member name and password for the account.
To use an activation key to create, upgrade, or renew a subscription:
Purchase an activation key through the Apple Store or an Apple Authorized Reseller.
Go to
www.me.com/activate and follow the onscreen instructions.
If you upgrade a subscription that is still within the free 60-day trial period, your one-year subscription begins immediately and any remaining trial period is eliminated.