How to create a Linkedin Application:

Please go at this url https://www.linkedin.com/secure/developer
and click on 'Add New Application'.

Fill in the required informations.
For 'Live Status', select Live.
Leave everything else by default.
Click 'Add Application'.

Copy the 'API Key' and 'Secret Key', click 'Done'.

Configure your linkedin account in the Social Accounts section.

Follow the link provided by MicroblogPoster to authorize your App posting on your behalf.