ASP.Net master page and content page

Use Master.FindControl function to find the MasterPage controls from a content page.
Try this:

Label mpLabel = (Label)Master.FindControl("masterPageLabel");
if(mpLabel != null)
{
    mpLabel.Text = "Your Text Here";
}

--Amit

I do not use Microsoft's controls for logins. In fact, at the professional level, nobody uses Microsoft's controls, but instead make their own user controls to fit their needs.

I think the root of you question goes to passing data from a content page to a MasterPage. Without more detail, I am half guessing on your needs. I would say that the login page can create a session (set a session variable) and all parent and child pages should be able to access your session variables.