Here's what you need to do:
1. After installing the WD software per the package instructions, open WDMyCloud Dashboard, which should be located on your desktop.
2. At the top of the Dashboard main page, click on Settings.
3. In Settings, ensure the General tab (on the left) is highlighted. Scroll to the bottom of the page.
4. Under Mac Backups, "Turn On" Time Machine.
5. Click on "Configure" to the right of the Time Machine On/Off slider.
6. If you have created multiple users (shares) for the WDMyCloud, select which user the Time Machine will backup to.
7. Using the slider bar, select the amount of MyCloud space you want Time Machine to use for backups.
8. Click Save.
Go to your Time Machine and Open Time Machine Preferences. Click on Select Disk, in the listings you should see a list of user names associated with WDMyCloud. Ensure you select the same user (share) you selected in step 6 above. DO NOT select Guest account, Time Machine backup will not work using the Guest account. If a password was created in the WDMyCloud Dashboard, you will be asked for the username and password at this point. (Both are listed in the WDMyCloud Dashboard under the Users Menu.) Time Machine will backup on schedule to your WDMyCloud as normal.
Good luck!