Hi there! Thank you for supporting the Kickstarter campaign!
I have actually already sent you an email regarding this issue last night. As stated in the Kickstarter update post containing the link to the backer-exclusive forums for signing up, the forums is only open to backers of tier 6 (Adept Mage, HK$157 / US$20) or above as part of their offered rewards during the Kickstarter campaign. If you believe that there's an error with the Kickstarter records about your pledged tier, please feel free to contact me at email@example.com so I can look into the issue further.
I apologize for any inconvenience or confusion caused. Thank you for your support and your message!