First, go to Settings > Add-ons and activate the networks. You can add several accounts for each, for example, your personal and company Twitter accounts.
Second, make sure that the user you’re logged in as has permission to post status updates to social networks. To do so, click on your name in the top right corner, or, if you’re an Admin, go to Account Dashboard and click on user’s name on the dashboard.
To post a status update, click on the + shortcut in top left corner and select Status Update. Select which of the active network accounts the update will be posted to and click Post.