To create a Dependency between components you can do so via the Component Details screen.  Click into a Component and navigate to the Dependencies Tab.

The screen is split into two sections, Current Dependencies and Potential Dependencies. Any existing dependencies on Components will be listed under the Current Dependencies section.

Any Potential Dependencies will be listed in the bottom section of the screen. You have the ability to add either a Predecessor or a Successor dependency.  When the dependency has been added it will move and sit under the Current Dependencies section. Once the dependency has been added a dependency connector will be drawn between the two components.