If your not able to change your status to Online or automatically are set to Away in the client when logging in, this can have two causes.

Primarily you need to ensure you haven't chosen to log in as Away.

If Auto away isn't activated when logging in this problem is caused by you already being logged in on another unit (computer, tablet or smartphone). Please make sure you've logged out on all other units before logging in at the unit you'd like to use at the time.