You can write a script to do it, using one of the following two methods:

Coroutines

Use a coroutine, something like this:

void Start()
{
     StartCoroutine(BlowStuffUp());
}

IEnumerator BlowStuffUp()
{
     yield return new WaitForSeconds(5);
     // Spawn explosion here
     yield return new WaitForSeconds(3);
     // Spawn more of them here
}

Invoke

Use Invoke(), like this:

void Start()
{
     Invoke("Detonate", 3);
}

void Detonate()
{
     // Spawn explosion(s) here.
}

Edit:

Since you edited your post and asked yet another question, you use Detonator simply by instantiating one of the prefabs it gives you. When a detonator object is spawned, it explodes, and then it automatically cleans itself up when it's done, so all you need to do is Instantiate() one of the prefabs.

Uncheck the explodeOnStart option, and then ,in your code, call the Explode() method whenever you want.