Multisite sub domain/blog created but "page not found" for wp-admin

I've just changed a single blog into a multisite blog.

I created a new user as a test to create a new blog.

The blog was created: http://greenpeony.com/momof2/

However the wordpress dashboard has no menu for choosing themes, adding a post etc.

When trying to add a post from the main blog/buddypress site I get a page not found error: http://greenpeony.com/momof2/wp-admin/

I'm sure it's something simple that I'm overlooking, but I can't seem to find the fix.

As a side note - I'm not sure I'm really looking for a multisite account.

What I do want is for registered members/users to be able to add to the main blog by creating their own content. However, I don't want to have to go through each user and give them permissions manually if that makes sense.

Is there a plugin that would do that? Am I better off sticking with multiuser?

Thanks in advance for any suggestions/help!
Ruth