The fourth step of the campaign creation process ('Contacts') allows you to select address books and/or segments to send your campaign to.
However, you can also access the same screen from Campaigns > My campaigns and clicking on the Send icon alongside the desired campaign.
On this screen, you're prompted to select the contacts to send the campaign to, and the default view will be your Address books tab.
Select the Segments tab, which will list any created segments.
Tick the box next to the segment you want to send to. You may select more than one and you may also select segments in addition to address books.
After having configured the other send options, click on Save & continue to continue the sending process of your campaign as usual.