There are several possibilities as to why you cannot ping between computers. I would have said it was a possibility that you were using NAT with your VMs, but that wouldn't explain why pinging works in one direction but not the other. Firewalls are another possible cause.
TBH, if you know enough about networking to use penetration testing software like Back|Track then I would have thought you were expert enough to solve this problem. The only solution I can offer is to set your virtual network cards to bridged mode.