Selecting, moving, and editing objects with the Selection tool
The selection tool is the most basic tool; it looks like the ordinary Mac OS X mouse pointer. 
Selecting objects
Click an object on the canvas to select it.
When an object is selected, the Style and Properties inspectors become enabled. You can use the inspectors to change anything about the selected object.
To select several objects at once, click in an empty area of the canvas and drag a box around the objects. If you hold Option, only objects entirely inside the rectangle are selected. Command- or Shift-click an object to add it to the selection or remove it from the selection.
Moving objects
To move an object, click it with the Selection tool and drag it to a new location. If you want to move a line, you might have to detach its end points from any objects they're attached to. To move an object straight horizontally or vertically, hold Shift as you drag. To rotate an object, hold Command and drag one of its selection handles; you can also hold Shift to restrict the rotation to 15-degree increments.
To move the selected object very precisely, press the arrow keys. Hold Option or Shift to move by larger steps.
If you have smart guides enabled, they appear as you drag to help you line up objects and space them evenly. Hold Command after you start dragging to temporarily enable or disable smart guides.
To drag a copy of an object instead of the object itself, hold Option as you drag.
Resizing objects
When you select something resizable, like a shape or a group, it gains eight selection handles. Drag one of these handles to change the shape's size. Hold Shift while resizing the object to maintain the aspect ratio— that is, to avoid squishing it. Hold Option while resizing an object to keep it centered on the same point.
If you select a table, the edge handles look like little grids. You can drag them to change the number of rows or columns in the table. Drag the corner handles to resize the table normally.
If you have smart guides enabled, they appear as you drag to help you size objects equally. Hold Command after you start dragging to temporarily enable or disable smart guides.
Extending the diagram
To create a child of the selected object, Command-click an empty spot on the canvas. To create a sibling, Command-Option-click an empty spot on the canvas. The new shape uses the same style as the original shape. The line connecting the shapes uses the style of a line already connected to the original shape; if no lines already existed, it uses the Line tool's current style.
Editing points
When you select a connection line or a custom shape, it shows the points that define its shape; these points appear as tiny diamonds.
Any line (created with the Line tool) has two points: a red one at its source and a green one at its destination. A line may also pass through any number of blue midpoints. Drag one of the points to move it; by dragging the source or destination point you can make the line connect to or disconnect from other objects.
Custom shapes (created with the Pen tool) and Bezier-type lines have control points at each vertex. Click a control point to select it; if a control point is hiding behind a selection handle, Option-Command-click to select it, or just select a different control point first to make the handles go away. If a control point has bezier handles, they appear when you select the point. Drag either of the two handles to adjust both, or Option-drag to adjust them one at a time. Shift-drag to constrain the handle's angle to 45-degree increments. Command-drag a point to give it handles. If you drag a handle close enough to the point itself, the handle disappears entirely.
The default way to add points to an existing line is to double-click the line, but this can be changed in the General preferences.
To add a point to a custom shape, double-click somewhere along the shape's stroke.
To delete a point from a line or a custom shape, select it and then either press the Delete key or select Delete from the Edit menu.