Going with the flow: How to get the water to the rain garden.
The simplest way to get rainwater to flow into your rain garden is to extend your downspouts and point them toward it. Since the rain garden sits in a low spot, the water will naturally flow toward it.
You could also run a length of pvc pipe underground from the downspout to the rain garden.
As a third option, you could install a swale running from your downspout to the garden. Start at the end of the extended downspout and dig a little ditch to the rain garden, making sure that it slopes away from your house and toward the garden. You can enhance the swale by planting turf or native plants, or lining it with rocks.