geoserver-exts currently depends on a recent version of geoserver master. You can build that first, or have the geoserver-exts build pull down the latest geoserver snapshot.

git clone git@github.com:opengeo/geoserver-exts.git
cd geoserver-exts
mvn install

1. In the parent geoserver-exts pom, add a new module to the list.

2. In the extension's pom, add a parent section to point to the parent pom.

    <parent>
      <groupId>org.opengeo</groupId>
      <artifactId>geoserver-exts</artifactId>
      <version>2.3-SNAPSHOT</version>
    </parent>