From the network admin you’re correct it will go to wp-admin/network/site-new.php.
Easiest way to see the issue is to go to the “My Sites” link and click “Create new site” from there (assuming logged in users can create sites). This is how non-network admin’s would create a new site.
This link is borked (unless you’re using “blogs” as the slug for “Site Activity” possibly… haven’t tested that). Here’s the trac ticket with the fix (slated for 1.7): https://buddypress.trac.wordpress.org/ticket/4632
The default BuddyPress theme there is a folder called buddypress/_inc/
This folder includes a file called global.js. That file controls the appearance of the post submission button. I checked and it doesn’t appear that your theme has that js file. I haven’t done much work with the template pack so can’t say whether or not it has this, but the missing js is why it isn’t loading.
You could get around this by adding the following to the bottom of your theme’s CSS:
`#whats-new-options { height: 40px; }`