We just got a new mini dachshund. He's about 12 weeks, now. He doesn't seem to want to get out of bed. He will eat and he poops and pees all right but he doesn't seem to want to get out of his bed, play, etc. He is also nervous; shakes all the time. I was worried that he might be sick, although the vet health certificate states differently. He isn't mean or in pain (I don't think) because he doesn't nip or bite. Help, he just won't "do" anything. Is this normal?

No, its not normal. I take it that the health certificate was from the breeder's vet. I would take him in to see your regular vet ASAP. The stress of recent weaning and changing environments may have stressed him enough for him to show signs of an illness that was not obvious when the first vet examined him. He may have just been poorly socialized, but you need to rule out physical ailments first.