This page have a few vidoes and pdfs that should help you get started.
But basically, you will need to run the Security Assist wizard to generate your login, registration, logout, forgot password pages, and you should be able to select the SiteAssist Pro template if you are using SiteAssist Pro for your site.
If you have the forms already created and you just want to use Security Assist to register or login a user, then you will need to use the Security Assist Server Behaviors, and those are located under (Window -> Server Behavior) and click the + button and then select Security Assist.
If you still have any questions, let me know.
I recommend that you run through the wizard on a test site first so you are familiar with the settings and the options in the wizard.