by clicking add/edit instance variable under instance variables in the object's properties-bar..
Yes that is the only option I can find, however that is not how it is done in the example.
In the example, the blob "number" value, auto increments every time you copy and paste an instance to the layout. So it seems to be an auto-generated instance variable. It only appears in the Properties but not in the Instance variables editor window.
I thought it may have something to do with Families but adding my sprite to a family didn't seem to make any difference.