I've read lots on this but can't figure out how to do it.

How would I create a code to send to people which they can then use in the shop? I can't find a discount / code/voucher module. I can see 'Gift/certificates/coupons' in the admin but it says it's not installed. How do I install it?

Once I have this working can I then have it so the discount only works on some products.