How are you setting up the details in the first page of the Security Assist wizard? I did a quick test of this scenario and I had no trouble doing what you are describing. In the Security Assist wizard I selected the SiteAssist template that all of my SiteAssist pages are derived from. I then selected the editable region, next I unchecked all of the pages except for the login. I used the folder icon to select the SiteAssist generated page and proceeded through the wizard.
In the end the wizard added the login form to my selected page.