The configuration menu can be accessed from within Media Browser using the wrench (or spanner) icon. Some themes will always show the wrench button on the Enhanced Home Screen. For example like this:
Some themes use a second menu like this:
which is followed by 
Other themes have the configuration menu hidden from view. In these cases you most often have to navigate to either the top or the bottom of your screen using the up and down buttons on your remote control.
As soon as you are at the top or bottom item press up or down once menu to show a hidden menu where you can find the configuration menu.
For example, in Vanilla you need can find the hidden configuration menu button at the bottom:
If you use Chocolate the hidden configuration menu button is at the top:
If you have “Use Transition Animations” disabled in Windows Media Center then you cannot access the config menu or config bars in most of the themes.
Configuration Menu or Media Browser Configurator?
There also is a “Media Browser Configurator”. The “Configuration menu” and the “Media Browser Configurator” are not the same.
The Configurator is used for setting up your Media Collections, install Plugins and such.
You can find the “Media Browser Configurator” in the Windows Start Menu under:
Start > All Programs > Media Browser > Media Browser Configurator