Steps to add Mopub or any library project as a jar / maven dependency in Android Studio (0.4.3) :
1 .. First, find out if this is any central maven repository available for your library if there is no need to download anything on your local computer. For example, actionbarsherlock is available on maven, so you just need to add this to your module build.gradle file in the dependency section.
compile 'com.actionbarsherlock:actionbarsherlock: 4.4.0@aar '
You can search and add the maven dependency on File>Project Structure > Modules > Dependency Tab > "+" Sign > Maven Dependency or search.maven.org.
2. If it is not available as a maven dependency (for example, in the case of mopub), you need to download the entire library project to your local computer. Since mopub uses maven and there is no gradle project repository available. The process is a bit panic.
First you need to create the jar file of your mopub SDK using Ant / Maven (I saw that the library has no resources, only java code, so the jar will work).
Now create a directory in the directory of the main module and name it libs (same as in eclipse) and copy the mopub jar here.
Go to File > Project Structure or right-click on the project and go to Open Module Settings and select "Modules in the window that opens."
Now select the module for which you want to add a dependency from the left pane, and click the Dependency tab in the right pane.
Press the Green button + key , place the window on the right on the dependencies in the vertical panel and select "File Dependency"
It will show your libs directory in the windows with your jar, select the jar and click "OK".
Doing this will simply add this line to the build.gradle assembly file depending on the dependencies so you can check.
compile files(libs/yourmopublib.jar)
You can achieve the same by adding this line manually to the build.gradle file, I would like to add it manually.
Sync your project with Gradle.
Done!
Note. Things are subject to change for future releases.