Only a Note shape can show description of an element when they are connected together with an Anchor. The palette of a mind mapping diagram does not have Note and Anchor by default, you can make them available by showing the category of a UML diagram, e.g. Use Case.
This is done by the popup menu of the palette and select the desired category as shown below:
Create a Note with ${modelDescription} as content. Drag the Anchor resource from Note to the mind mapping node:
And here is the result:
Alternatively, there is a simpler way that can show description of any selected shape regardless of diagram type, just click the bottom left Show Description button in the status bar of diagram editor. You can also update the description here directly.