My 10-yr-old son and I are new mBot users. After some exploring, we are successfully using mBlock on a Mac running El Capitan to program the mBot.
After installing mBlock and connecting the mBot to your computer, you won't see the correct connection under Connect>Serial Port because you have to install the mBot driver, and there's an extra step required to do this installation if you're running Yosemite or El Capitan. Here's what you do:
Go to the official mBot driver page: https://github.com/Makeblock-official/Makeblock-USB-Driver
Click the Download ZIP button on the right.
This will download a folder called: Makeblock-USB-Driver-master
Double-click on this zip file in that folder: mBot_Driver_for_MacOSX.zip
This produces an installer called: CH34xInstall.pkg
Ctrl-click on this and choose Open. Then continue with the install.
The driver still won't work because of security measures built into OS X Yosemite and El Capitan, so you need to follow the instructions at the bottom of that GitHub page where you downloaded the driver. For El Capitan, restart and press CMD+R as soon as you hear the startup sound. This puts the computer in recovery mode. Under Utilities, choose Terminal. Then enter this command in the terminal window, followed by return (single space after enable):
csrutil enable --without kext
Then do a normal restart (no CMD-R).
Now, switch mBot off and disconect the battery power. Launch mBlock on your Mac. Plug USB cable to mBot and computer. You should see LED on mBot showing USB power. Turn mBot switch on (more LEDs light). A window for opening Network Preferences should appear on your Mac. Click Cancel. Now when you go to Connect>Serial Port you should see this second option:
Choose that option and you now have a connection to your mBot that enables you to upload your Scratch program to the mBot by clicking on Upload to Arduino. You get a progress window when you do this. When done, switch mBot off, disconnect USB, reconnect battery pack, and switch on.