By default, Mokai is configured with an embedded Derby database. To use MongoDB you will need to modify the <MOKAI_HOME>/conf/core-context.xml file. Uncomment the MongoDB line and comment the Derby line.

...

<!-- <import resource="derby-context.xml" /> -->
<!-- <import resource="mysql-context.xml" /> -->
<import resource="mongodb-context.xml" />

...

Configure the MongoDB URI in the <MOKAI_HOME>/conf/mongodb-context.xml file.

That's it!