After you enroll in a course from the edX course catalog, the course is listed on your dashboard at edx.org.
You will find the start date below the course name.
To begin click View Course. (If the course start date hasn’t passed, the View Course button won’t be available.) Your course opens to the Course page. You may then click Start Course in the upper right or scroll down to select a section from the course outline to begin viewing the course lessons.