Add the following in the build gradle file in the dependencies part:
compile fileTree(dir: 'libs', include: '*.jar')
the above assumes that the library is inside the libs
folder of your project.
Alternatively you could also add the dependency like this by telling gradle a single jar file:
compile files('libs/yourlibrary.jar')
After having added this lines you should sync your project again and android studio should allow you now to import those classes from your library.
Update: Actually the jsoup library is available in the mavenCentral repository. So a nice way to include this dependency is:
compile 'org.jsoup:jsoup:1.8.2'
or just do this to always take the latest version:
compile 'org.jsoup:jsoup:+'
Note: you need to have mavenCentral repository added in the build grade if you do it like this:
repositories {
mavenCentral()
}