Adapters like Oracle, SAP etc. are free for Biztalk, but dont get installed when you install BizTalk Server 2006. You'd need to download them from the microsoft website. They are included in a package, which contains the Oracle adapter as well. Once you download that package, follow the instructions on this link to install and configure them:
http://download.microsoft.com/download/2/4/c/24c19d0b-4f00-4c46-b74c-acc02988d4a0/Installing and Configuring Microsoft BizTalk Adapters for Enterprise Applications.htm
Regards,
Sajid.