Dell Aero Unlock

I wish to install on my Dell Aero non-Market apps. When I attempt to do so, I receive a message stating "Install blocked. For security, your device is set to block installation of applications not sourced in Android Market." Selecting the Settings button on this message opens the Application Settings screen, which lists Manage Applications. Where is the setting to permit me to install non-Market apps? How do I change these settings? Arbab Rafiq