I think the newsletter is mainly when Mena has notices for anyone participating in the Sew Weekly forums/challenges; i.e. when she sends out information to start the sew alongs in the beginning of the year, etc. That's just my guess as I've received a couple of messages from her in the time that I've been a member of the sewing circle. The best way to follow the blog is via the RSS feeds. Mena has more details of that on the main site under the 'Feed' navigation tab. If anyone else has a different definition of the newsletter, please feel free to correct me!
There used to be a regular weekly newsletter in the very beginning but it didn't work out on a weekly basis and now it's just 'as and when'.