Assembly is done using the provided Assembler:

public void assemble( ModuleAssembly module )
    throws AssemblyException
{
  [...snip...]

    new MongoDBEntityStoreAssembler().withConfig( config, Visibility.layer ).assemble( module );
      [...snip...]

}

Here are the configuration properties for the MongoDB EntityStore:

public interface MongoEntityStoreConfiguration
    extends ConfigurationComposite
{

    @Optional
    Property<String> hostname();

    @Optional
    Property<Integer> port();

    @UseDefaults
    Property<List<ServerAddress>> nodes();

    @UseDefaults
    Property<String> username();

    @UseDefaults
    Property<String> password();

    @Optional
    Property<String> database();

    @Optional
    Property<String> collection();

    @UseDefaults
    Property<WriteConcern> writeConcern();

    enum WriteConcern
    {
        /**
         *  Write operations that use this write concern will wait for acknowledgement,
         *  using the default write concern configured on the server.
         *  This is the default value.
         */
        ACKNOWLEDGED,
        /**
         * Write operations that use this write concern will wait for acknowledgement from a single member.
         */
        W1,
        /**
         * Write operations that use this write concern will wait for acknowledgement from two members.
         */
        W2,
        /**
         * Write operations that use this write concern will wait for acknowledgement from three members.
         */
        W3,
        /**
         * Write operations that use this write concern will return as soon as the message is written to the socket.
         * Exceptions are raised for network issues, but not server errors.
         */
        UNACKNOWLEDGED,
        /**
         * Write operations wait for the server to group commit to the journal file on disk.
         */
        JOURNALED,
        /**
         * Exceptions are raised for network issues, and server errors;
         * waits on a majority of servers for the write operation.
         */
        MAJORITY;

    }
}