You can login to http://clanforge.multiplay.co.uk with your email address and password that you specified during your order/registration process.
If you can't login, first ensure your account is activated by checking your inbox and spam folder for any activation emails.
If you were under the impression you had purchased a Clanforge server using your email address we would suggest checking any alternative email accounts you may use.
Once you have activated your account and you still can't login, try resetting your password over at http://www.multiplay.co.uk/account/recover
If this fails, please create a ticket at https://support.multiplay.co.uk/support/tickets/new and be sure to include either your MH number, Order Reference or InvoiceID and your billing address for verification in the ticket for faster ticket resolution.