1) Set a Break Point first, then right click again and select Break Point Condition. In the expression editor, create an expression such as [My Value]=10 to define the condition. See the Help for more explanation of how the condition will be met.
2) Pre and post conditions are unrelated to break points. They are for documenting a design principle where you state 'what must be true before this page starts' and 'what will be true after this page ends'.
------------------------------
John Carter
Professional Services
Blue Prism
------------------------------