Google is now offering the opportunity to migrate to new Google maps. (ref: http://googlesystem.blogspot.com/2013/05/the-new-google-maps-now-available.html)
However one disadvantage is that "My Places", which includes Google Custom Maps, is not part of the new Google Maps interface.
At the moment, you can temporarily switch to the old interface by clicking the "options" icon in the black navigation bar and
- Selecting "My Places" OR
- Clicking the "classic maps" link.
Or you can get to them by navigating to https://www.google.com/maps/myplaces
From here you can see and edit your existing Google Custom Maps, but you cannot create new ones. To do that, you need to use Google Maps Engine Lite - which was announced recently.