You should just click "Start" but I see that is greyed out. In sharing preferences you can choose what you want to share over the service, such as all folders or just your public folder and screen sharing.
I never had a problem turning it on, but perhaps your network is blocking it being turned on ?
Perhaps you have to have to computers registered with the same account (ie. not a sub-account and a master-account) and so if you only have one registered with your sub-account (in sync preferences) it wont let you turn it on as there's nothing to connect with..?