I'm trying to set up my mailboxes in Email Center Pro to get email messages coming in. However, I don't see a way to set it up to use IMAP. Am I missing something?
You're not missing anything, Email Center Pro does not support the IMAP protocol as it would be too resource intensive. To get email messages into ECP you should either set your mail server up to forward a copy of the messages to your ECP mailboxes or set up a POP server configuration in ECP to pull the messages down from your server. You can read more about setting each one up here: http://www.emailcenterpro.com/help/content/account/get_email.htm
Get email into your mailboxes by setting up mail forwarding or by configuring a POP server.