I was also facing the same problem. Add this to your pom.xml
file (after <repositories>...</repositories>
for instance):
<pluginRepositories>
<pluginRepository>
<id>flex-mojos-plugin-repository</id>
<url>http://repository.sonatype.org/content/groups/flexgroup</url>
<releases>
<enabled>true</enabled>
</releases>
<snapshots>
<enabled>false</enabled>
</snapshots>
</pluginRepository>
</pluginRepositories>
Solved it for me :)
Some IDEs will generate a POM file already containing this plugin repository.