If you don’t know what you are doing you an easily brick your My Cloud using SSH or render it crippled. Using SSH can void the My Cloud warrantee. So use at your own risk.
To SSH from a Mac, from the “Terminal” type:
ssh root@< My Cloud IP address>
So in your case if the IP address of your My Cloud is 10.0.0.3 you would type in “ssh root@10.0.0.3”.
You will then be prompted to enter a password. The password is: welc0me
(Note: that is the number zero not the letter “O” in the password.)
On the latest firmware the My Cloud will prompt you to enter a new SSH password at the first SSH login to the device.
Edit to add: El Capitan is known to have problems with the My Cloud. See the many other thread detailing various problems using El Capitan to access the My Cloud. For example this recent thread:
https://community.wd.com/t/support-os-x-el-capitan/96744/117
