You have to point Gradle to the location of your plugins so Gradle can find them.
Do this by adding a repositories { ... }
to your build.gradle
.
Here's an example of adding three repositories, JCenter, Maven Repository, and a custom repository that offers dependencies in Maven style.
repositories {
// Adding these two repositories via method calls is made possible by Gradle's Java plugin
jcenter()
mavenCentral()
maven { url "http://repository.of/dependency" }
}