I did find the Nissan map update site late last month on an early attempt. The NAVTEQ link eventually also led me around to Nissan's site. That let me specify the Leaf and model year, which took me to a page on the Nissan Nav Shop and prompted me to download the Map Update Tool, which took me to a page with a non-working download link for Windows. Inspecting the html source, it LOOKs like the thing is pointing back to TomTom, so I hand entered the address from the source HTML and downloaded InstallMapUpdateTool.exe from TomTom's download site. On execution, it installs a program declaring itself to be the Nissan Map Update Tool - which won't work without a USB stick that has been downloaded from the in-dash Navigation & Entertainment Console in the Leaf. That part was pretty straightforward - might be wrong, but easy.
Unfortunately, all the instructions I've found on the details of how to get that crucial first step of downloading the existing maps from the Leaf's Nav system TO the USB stick have hit a brick wall. I can't find any path so far to the options that are supposed to let me prep the USB stick in the car.
I popped the SD card out of the console and it has the information 25920 5SA2A USA/CAN. I poked around the Nissan accessories site and found that I could order an SD card with 25920 5SA2B - which MIGHT be the later version, but it's $350 US and if I can get a free download it's worth banging my head against the wall a bit more. I have a very hard head.