At first you have to create your Ext-configuration with JavaScript. For more information look at ExtJS Integration=>Quick Start. Don't forget to add your used controller to your Acl.php.

The important step is this:

There is a global JavaScript variable called "Welcome" which contains an Ext-configuration of a panel to show in backend at /admin. (That's normally a box with your web-name and the Kwf Version) Just asign Welcome your dashboard variable.

For example you did the Quick-Start you have to add this line at the end of your JS file:

Welcome = Example.Foo;