Possible Duplicate:

How do I add a user to the “sudo” group?

I've added myself back into the admin's group I think, by following this.

But I still can't do commands like:

sudo aptitude update

I get the message

oshirowanen is not in the sudoers file.  This incident will be reported.

How do I add myself into the sudoers group now that I am in the admin group?