Where should I hook into admin?

Not sure about the Membership plug-in bit, but admin_init will be your best bet. I assume that you don’t actually what to hook into the admin header though – this would be admin_head or admin_head-(plugin_page) – but you cannot redirect after these hooks (headers already sent).

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)