After you have designed tasks and alternative tasks in the SeeYou's own Task editor, you have to assign the task to the Contest day.
You can do this by choosing Edit > Assign Task from the main menu. Double click on a task to close the dialog. This way the task for a particular day is selected or changed.
Alternatively you can drag and drop the task from the upper part of the Tasks window (where all tasks are listed). You should drop it into the upper part of the Contest window (where there is information about the task), observe the Icon to see where to drop it.