Support Forum

I'm trying to find a way to force every member/user of my simple:press forum to receive a weekly email with that week's new topics/posts.

You will find that in most countries (maybe even the majority now), it would be illegal to 'force' emails upon your users. They must be able to unsubscribe and should, be given the option to opt in (subscribe) in the first place.

I see under admin>components>subscriptions>force digest emails(if enabled) but how can I enable this option  ?

You click on the check box to enable it. it means it will use that setting if digests are enabled above that option. The popup help explains more.

I'd ideally like every user to see a snippet of every new topic posted in a single week via forced email and for them to not be able to unsubscribe.
Is this possible ?

See my comment above about forcing emails on your users.