You have to! When you connect to the Rafflecopter Facebook app to set up your giveaway, it will ask you to login with your personal account. What will then happen is the app will recognize the Facebook pages that you're an admin for so you can run a giveaway on one of those pages.
It's not possible to login with a Facebook page account, only a Facebook profile. This is a limitation on Facebook's behalf and is the only way to connect any app to your Facebook page.
For additional help on setting up your Facebook giveaway, here's a blog post that describes how to set up a Rafflecopter giveaway in 5 easy steps.