How to set a z-block in a custom position?

You can stick to the following instruction while using custom position feature:
- create a z-block;
- set Block Position to Custom;
- set Custom Position to any unique ID, for example, "my-custom-z-block-position" (without quotes).

Then add the following code into CMS:
{{block type="zblocks/block" position="my-custom-z-block-position"}}

or in XML: