The link to "Open Exercise System" is only found on the "Exercise" page of a lesson. In this course, there are some lessons that do not have exercises, so you will not see this "Exercise" page. However, for all lessons that do have an exercise, you will see the "Open Exercise System" link on the associated "Exercise" page.
There is no global link as you will get a unique instance (link) of the exercise environment. To acess the "Open Exercise System" link, you must navigate to the actual exercise page for each lesson. For example, in the lesson "Operators, Portals and Cases" you will find the link to open the exercie system on the exercise page. Notice at the bottom of this screen shot there is the "Open Exercise System" link.