Can't post in Bioware Social Network forums

Might be this:

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Bioware Social Network - Rules of Conduct

13. Only users in good standing may post in the forums. Our forums are a place for responsible and respectful conversation about video games. To ensure they stay that way, we’re only allowing users with accounts in good standing to post on this site. An account is determined to be in “good standing” in the following ways:

• For users who have not already logged into an EA game using this Origin account, the account must be older than 30 days;
• For users who are already playing EA games and have logged in using this Origin account, the account must be registered with this site for at least 24 hours.

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Have you registered your game?  Posting in the ME3 MP forum requires game registration.  Using the side bar, go to My Games-> Register games.  http://social.bioware.com/user_bw_games_register.php  

If ME3 has not been registered, that is your problem.

I was having the same problem as OP and your answer was pure gold. How did you even find that page? It's so buried!

Thanks!