Adding Skype usernames to OS X Address Book (workaround)
OS X's Address Book only allows adding AIM, Jabber, MSN, ICQ and Yahoo IM addresses to contacts. This has been a known issue (Apple bug #4270469) for quite a while, and there is no indication of it getting fixed anytime soon.
You can work around this by adding an URL field to your contact template in Preferences and using the "skype" schema (urls like skype:username, skype://username or even callto://username). The custom URL field can also be named any way you want, so you can set it to "Skype" (default is "home page").
Don't expect any fancy integration in the Skype client, like showing contact names from Address Book instead of your contacts' own defined names, but at least you can call them from the Address Book with one click.
Step by step
Open Preferences -> Templates. Select Add Field -> URL or click the + next to an existing URL field, if you already have one. Change the name to "Skype" or something similar by clicking on it, selecting "Custom..." and typing it in.
Add URLs like "skype:username" to the newly created field for your contacts.
