Try adding this to your build.gradle file
dependencies {
// other dependencies
// ZXing
// Supports Android 4.0.3 and later (API level 15)
compile 'com.journeyapps:zxing-android-embedded:2.0.1@aar'
// Convenience library to launch the scanning and encoding Activities.
// It automatically picks the best scanning library from the above two, depending on the
// Android version and what is available.
compile 'com.journeyapps:zxing-android-integration:2.0.1@aar'
// Version 3.0.x of zxing core contains some code that is not compatible on Android 2.2 and earlier.
// This mostly affects encoding, but you should test if you plan to support these versions.
// Older versions e.g. 2.2 may also work if you need support for older Android versions.
compile 'com.google.zxing:core:3.2.1'
}