DEVONthink and DEVONagent, like most other Mac OS X applications, make heavy use of toolbars. The tools give you immediate access to frequently used functions such as for navigation, switching views, creating and deleting items as well as searching. But there are many other tools available for you that you don’t see on a first glance. Choose View > Customize Toolbar (you can also find this command in the contextual menu when you right-click the toolbar itself) to open the toolbar editor. Drag any of the available tools to the toolbar, rearrange the tools by dragging them around, or remove tools by dragging them from the toolbar into the open. If you want to get back to the default tool set, drag it from the bottom to the toolbar, effectively replacing all changes.
BTW: This is the very same in (almost) every Mac OS X application that uses toolbars.