If you see the error 'no module named microbit' or similar when running the Mu beta version (as available in the Raspberry Pi APT package repository), you may need to switch the editor to Microbit mode. Plug in your micro:bit and follow this guide.

Mu v1.0 

A mode button has been created as the first icon in the menu bar. Click on the Mu logo to open the mode selector dialogue where you can choose BBC micro:bit mode from the menu

Mu supports many devices running different versions of Python or MicroPython, the mode selector allows you to easily switch between these.