Steps to add Mopub or any library project as jar/maven dependency in Android Studio(0.4.3) :
1. First search if their is any maven central repository available for your library if it is then no need to download anything in your local machine. For example actionbarsherlock is available on maven so you just need to add this in your module's build.gradle file under dependencies section.
compile 'com.actionbarsherlock:actionbarsherlock:4.4.0@aar'
You can search and add maven dependency from File>Project Structure > Modules > Dependency Tab > "+" Sign > Maven Dependency
or search.maven.org .
2. If it is not available as maven dependency(like in case of mopub) you need to download the whole library project in to your local machine. As mopub is using maven and there is no gradle based project repository available. The process is little bit panic.
First you have to create a jar file of your mopub SDK using Ant/Maven (I have seen there is no resources in the library, only java code so jar will work ).
Now create a directory inside your Main Module's directory and call it libs(same as in eclipse) and copy your mopub jar here.
Go to File > Project Structure
or right Click on project and Go to Open Module Settings
and Select Modules in opened Window .
Now Select the module for which you want to add the dependency from left panel and click on Dependency tab on right panel.
Click on Green "+" sign button, placed right to dependency window in a vertical bar and choose "File Dependency"
It will show your libs directory there in a windows with your jar, select the jar and click Ok.
Doing this will just add this line in your module's build.gradle file under dependency so you can verify.
compile files(libs/yourmopublib.jar)
You can achieve the same by adding this line manually in build.gradle file as well, I like to add it manually.
Sync Your Project with Gradle.
Done!
Note : Things can be changed for later releases.