Install from unknown sources on Nook Tablet

I am using a rooted NOOK Tablet with Android Gingerbread, and am installing an application from an email. I am using the NT Hidden Settings application to enable the installation of apps from unknown sources. I check the box and click OK knowing the dangers of 3rd party applications. I go back to the email, click the attachment, and when I hit the installation screen, after I had just checked it, it says install blocked. I returned to the setting and the check box I had filled in was now empty again, and I am caught in the loop forever. What can I do to make the app install?