Below is a video that will show you how to turn the LinkedIn Signal on to indicate you are open to new opportunities. If you are in a confidential Job Search, this is a a safer way to notify Recruiters that you are in a job search. Only Recruiters with a Recruiter account with LinkedIn will be able to see this signal when they hover over your Summary.
NOTE: No one else will see this signal. However, one possibility is that a Recruiter or HR person within your company could go look at your profile and hover over the Summary section and find out you are open to new opportunities. So consider if you think this is possible before turning this signal on.