If you want to create the application bundle as a part of Maven's build cycle you must configure the plugin in your POM.
This example show the simplest possible configuration. The <mainClass> configuration property isn't required, the plugin will try to find a main class in your classes and dependencies. However, it's probably smart to be explicit about which class to run.
<project>
...
<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>osxappbundle-maven-plugin</artifactId>
<version>1.0-alpha-1</version>
<configuration>
<mainClass>com.example.Main</mainClass>
</configuration>
<executions>
<execution>
<phase>package</phase>
<goals>
<goal>bundle</goal>
</goals>
</execution>
</executions>
</plugin>
...
</plugins>
<build>
...
</project>