I'm a novice, but I'll post a comment anyway, 'cause everyone has been so helpful to me:
At the top of the 'send mail' dialogue, there is the option to 'send one email using data from the current record' or '(multiple emails) one for each record in a found set.
Maybe clicking the second option will help you, then you can specify the field that contains the email address.
If this works, you'll be the first person I've helped in this forum! (good luck!)
Doh, it is always something so simple.