Opt-in Optin on GF for MC?

I read this doc: http://www.gravityhelp.com/documentation/page/MailChimp_Add-On and have successfully set this up per the instructions. What I'd like to know is if there's a way to add a check box on the GF that opts them in rather than them getting automatically sent to MC. Then the double opt-in is setup to happen. But I'd like them to CHOOSE if they want to signup when they complete the contact form rather than automatically being signed up and having to basically receive the opt-in emails when they may not have wanted them. They'll seem pushy. i thought there'd be a field to add for MC in GF but I don't see it. Am I missing it or trying to do something unusual? Thanks.

I'm sorry...I feel a bit dumb here...but I have NO idea where you're talking about! I don't see this in GF. Are you talking about somewhere in MC? Sorry to be so lame.

Thanks...got it now. I never was notified of your second response (I was for your first). Anyhow, I just hadn't added a drop-down choice so while I had activated the double opt-in there was no condition it indicate it. Thanks.