In order for a lead to show up in your Pipeline Report, it will need to have a Pipeline Rating, and a Followup Date set.
The easiest way to remember these two things is to select Log Appt under a lead's profile, and adding your Pipeline Rating and Followup Date. By logging your appointment with these set, you'll automatically move the candidate into your Pipeline Report.
To remove a lead from your Pipeline Report, clear their Pipeline Rating by opening the lead's profile, clicking Edit under the Pipeline section, and clicking the X on the Rating dropdown. See Viewing and Using Your Pipeline Report for more details.