I have loaded the BBC MicroBit extension. All the services are there - accelerometer, button, magnetometer, temperature etc - EXCEPT the Uart service. Where is it? can't find the option anywhere! Very confused 
Bumping this - I have the same issue. I imported the extension via URL (http://iot.appinventor.mit.edu/assets/resources/SimpleMicrobit.aix) and get all the components listed in the documentation Except Uart, which is the one I'm most hoping to play with, for some of the new hardware and for attached hardware.
Please let us know if there's another way to access UART.
I found another thread (UART with Micro:bit and App Inventor) that helps with this, referring to a different extension, found at:
http://iot.appinventor.mit.edu/assets/resources/com.bbc.microbit.profile.aix
I take it this is the newer of the two extensions I've found? Is it linked somewhere that I missed?
This seems promising.
Actually, that's the link to the first iteration of the extension before Farzeen and David hacked on it to create the SimpleMicrobit version that is published on the IoT website. I've since restored access to the UART and made some modifications, but we haven't officially published it. You can download this preivew version from Dropbox:
Thank you for this, Evan! I'll see what I can do with this extension. Are there any known gotchas?