Once you have inserted an arrow between two instructions, you can change
its condition - see Arrows and conditions for more
information. From the arrow's context menu - you get it when you right-click
on the little circle - select Properties. You'll get the arrow's
properties dialog where you can modify the condition with the menus: the
Left operand menu where you can select a variable or nothing to
be the left operand and the Right operand(s) menu where you can
select the symbols and the variables for the right side of the condition.
The "NOT" Condition check-box shows if the condition is
negated.