Massage of the tummy can help - massaging clockwise around the babies tummy.
Here's a youtube video I really like.
http://www.youtube.com/watch?v=vseuKsNGmXsYou can also press on pressure points on the soles of their feet to help relieve constipation.
If the baby isn't distressed, then I wouldn't worry too much at the moment, as they can often go 7-10 days before pooing.
But definitely increase their water intake if possible in the formula feeds.
If they are distressed, ie. pulling knees up, tummy distended, not feeding, inconsolable
Then it might be worth a trip to OOH/ A&E for a suppository.
We did this with Eleanor when her tummy blew up so swollen & she stopped feeding - one suppository later, and HUGE explosion and she was starving!
If its happening alot with the formula they are using, then it might be worth trying a different brand
Hope that helps