Email Campaigns is a module included on the plans that bundle marketing tools. If you don't see it in your sidebar, it isn't part of your current plan yet.
How to check
Open Settings → Modules. If Email Campaigns is available on your plan you can turn it on there; if it isn't, you'll see an Upgrade prompt.
Sending from your own domain
Everyone with the module can send (from a shared Studio address by default). Adding your own verified sending domain — for a fully branded from-address and the best deliverability — is a top-tier plan feature. See Sender identity & sending domain.
Upgrading
Go to Settings → Billing to move to a plan that includes Email Campaigns. Your People, lists, and any drafts carry over instantly.