I just a brand new WD MYCloud to replace my old one. How do I get my account back up?

What “account”? The Users account and their passwords, the Shares or the online account?

While one could possibly use the WD My Cloud dashboard to export the System Configuration to a file that can be imported to a similar My Cloud running the exact same firmware, the System Configuration file does NOT contain the User accounts or Share information. Or one could use the Safepoint option on v4.x My Clouds, or v2.x My Clouds to backup/move the contents of One My Cloud to another. But one would have to do so between My Clouds running similar firmware versions. In other words from v4.x My Cloud to v4.x My Cloud or from v2.x My Cloud to v2.x My Cloud.

Otherwise, generally one will have to recreate the User accounts and passwords along with any Shares on the new My Cloud. There may be other ways to move the User accounts but it would likely involve using SSH to copy config files between similar Firmware devices.