Depend on the macro in your project's maven-site-plugin configuration like this:
<plugin>
<artifactId>maven-site-plugin</artifactId>
<version>2.1.1</version>
<dependencies>
<dependency>
<groupId>org.flobakk.doxia</groupId>
<artifactId>yuml-doxia-macro</artifactId>
<version>1.0-SNAPSHOT</version>
</dependency>
</dependencies>
</plugin>
(The macro is not yet available in a repository, so you need to download the source, and mvn install it to your local repository, or even deploy to your private repository, first.)
Now, in your APT sources you can invoke the macro by using %{yuml|model=...}. E.g. to make this model:
use %{yuml|model=[yuml-doxia-macro]speaks with->[yuml.me]} in an APT document. For the full diagram syntax accepted by yUML.me, and hence the yuml-doxia-macro, see http://yuml.me/diagram/scruffy/class/draw. The macro is set to create class diagrams by default, but can also be set to create activity or use case digrams by using the type parameter, e.g. %{yuml|type=activity|model=...} or %{yuml|type=usecase|model=...}.