For now, you can't (officially). Unfortunately branded products like skype, microsoft, etc don't directly translate into chromebook usage and generally, google will come up with their own knock-off programs. Because chromebook is a google product, google already has google chat - so that is supposed to replace your need for skype.
I have not tried to put Skype on my Chromebook as it has Google Talk which allows you to call US and Canada free. My guess is you may not be able to download Skype due to the lack of space on the Chromebook hard drive.