It looks like you've either lost your connection to the server or something has produced an error. If you're still connected to the server, you might want to check your Dagobahd logs to see if you can identify the problem.
Manipulating the DAG Directly
To add a new task, scroll down below the graph to the "Add Task" section.
To add a new dependency, just click and drag on the graph. Start at the independent node, and drag to the dependent node. In other words, if task A needs to finish before task B can run, then drag a line from task A to task B.
To delete a task or dependency, first click on it to select it (it will become dashed). Then hit "d" on your keyboard (your mouse needs to be over the graph for this to work). You can also delete tasks in the table below.