You have to use a Smart List to report on things like this.
Visits Web Page IS "X" IN PAST "180 days"
What should I put in "Flow". I cannot activate it without something in "Flow". Thanks.
If you create a Smart List in the Lead Database using Josh's suggestion, that will give you a list of leads who visited your landing page.
Sounds like you've tried to set up your list in a Smart Campaign in Marketing Activities, where you are required to enter something into a Flow in order to activate.
Hope this helps,