You would run the sql statement on your DotNetNuke database using a tool like Query Analyzer. Before doing this, you probably want to query the database to see if you already have this module defined. You can use this statement to only insert the record in question if it does not already exist:
IF NOT EXISTS (SELECT * FROM Modules WHERE ModuleDefID = 91) INSERT INTO Modules (ModuleDefID, ModuleTitle, AllTabs, IsDeleted, InheritViewPermissions, Header, Footer, StartDate, EndDate, PortalID) VALUES (91, 'Windows Authentication', 0, 0, 1, NULL, NULL, NULL, NULL, 0)
If you already have this record in your Modules table, you have some other issue. Many times, people will set their portal's login page to be the home page. If there is no account login module on the home page, you are out of luck because you are directing people to a page without a login module. If you think that's the case, search around these forums. There are MANY solutions to this issue. I hope that helps.