Website and forum are different systems, requiring different login/registration.
If you purchased RM in the forum store, it should be in the content area of your profile.
If you purchased RM by the main site store, the serial had been emailed to you.
In both cases, you can send a support ticket with the email used to purchase the program to have the people from Degica check everything and (in the case of the main store) resend the email with the activation code.
However, if you don't get a passwort mail when trying reset passwort on the main site, then most probably you only registered and purchased by the forum store. In that case, your serial will be in the client area (one of the links of the menu under your name when logged-in), then purchases.