First of all you can use this library as dependency simply adding to your module/build.gradle this dependency:
dependencies {
    compile 'com.github.navasmdc:MaterialDesign:1.5@aar'
}
If you want to import locally as module, you have to add this part in your module/build.gradle: (this part is added in the top-level file in the library)
buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:1.+'
        classpath 'org.codehaus.groovy:groovy-backports-compat23:2.3.5'
        classpath 'com.jfrog.bintray.gradle:gradle-bintray-plugin:1.0'
        classpath 'com.github.dcendents:android-maven-plugin:1.2'
    }
}
Pay attention that this library is not updated with the last support libraries, and doesn't use the new design support library.