Click the plus icon in your node Properties. Other options include Edit (Pencil), Delete (X), and move Up or Down in the list.
You must give your Variable a name (beginning with an alpha character), select the Type of variable and then set a beginning Value.
Variable types are Array, Boolean, Date, Double, Integer, Long, String and XML. The beginning value will default to an empty string on the String type or zero for any of the numeric types.
You can reference previously defined variables to concatenate into your new variable or you can reference Landmark Configuration set global variables (Properties).
To select an existing variable, key [Ctrl] + [Spacebar]. Variable values that are from Landmark Configuration sets are prefixed with _
Even if your flow is associated with a Service, you will find the need to define variables for use within the flow. These types of variables are often referred to as persistent since they will retain their value throughout the processing of the WorkUnit.
You can update your variables using the Assign node. But that's a topic for another day.