The forums were recently targeted by a group of spammers, creating hundreds of new accounts and posting several messages under each account. To stem this flow of spam, we enabled approval for new accounts, however with many new accounts still being created by the spammers every day it is quite easy for legitimate new users to be lost in the noise. If you have registered recently and are now unable to login, please send an email to admin@monogame.net with your username and we will approve your account as soon as possible.
Thank you for your understanding, and we will endeavour to get better anti-spam measures in place soon.