I saw this still didn't have the answer OP originally wanted, so despite it being a bit older, here's for future Googlers:
Download the jar from https://developers.google.com/mobile-ads-sdk/download
Put this in your pom.xml (with the appropriate version of course):
<dependency>
<groupId>com.admob.android</groupId>
<artifactId>ads</artifactId>
<version>6.4.1</version>
</dependency>
And then run this on your shell, again with the appropriate version/jar name:
mvn install:install-file -Dfile=GoogleAdMobAdsSdk-6.4.1.jar -Dversion=6.4.1 -DartifactId=ads -DgroupId=com.admob.android -DgeneratePom=true -Dpackaging=jar
Mind you that the above command has the version that you need to change in two places - once in the filename, and once in the -Dversion parameter.
It might also be a good idea to include the latest version of the library in your project and have something like the following in your pom.xml above the dependency, as this isn't a step you are likely to remember if you had to google it ;)
<!-- If you just set up your dev system and the following dependency gives you an error,
run these commands:
cd project-root
mvn install:install-file -Dfile=libs/GoogleAdMobAdsSdk-6.4.1.jar -Dversion=6.4.1 -DartifactId=ads -DgroupId=com.admob.android -DgeneratePom=true -Dpackaging=jar
-->