Add content links for Organic Groups
Make sure you also install the eva module, in addition to clearing your caches via admin/config/development/performance.

Also, og_extras only works with og 7.x-2.*.

For the "create content" block, look  for "Node content links" under blocks. You also need to configure the Organic Group module to work with the entityreference_prepopulate module, to make it work properly. See here for guidance.