As you may know, in version 7, the Login Form Widget is no longer needed because there are native authentication widgets. A brief introduction to how to use these is outlined in the Mendix 7 release notes, I have included an excerpt below:
Are you experiencing problems using the login widgets? If so, could you provide some more details so the community can try to help?
In order to use a custom login page, in the security for your project, you need to have security turned on and Anonymous users enabled (I always create an Anonymous user role for Anonymous users, to make sure that I can control specifically what anonymous users see). I have included a screen shot below:
Once you do that, for each Navigation Profile, you will see an option to select a sign in page for authentication, as shown below. In this box, you can select the custom login page you created.