Using Address Book in Lion I can't edit existing cards: the cards don't show the "Edit" button, and Edit>Edit Card in the menu is disabled. My workaround is to click "Add card" and then use the forward/back arrows, but this is does not meet my usability expectations for Mac OS.
Try rebuilding the database file Address Book uses.
To do that, quit Address Book, then while in Finder hold down the Option key and click the Go menu - select Library from the list. That will open the hidden Library folder in your user home folder. In that folder, locate and open the Application Support folder, then the AddresBook folder. In that folder locate the file named AddressBook-v22.abcddb and throw it away.
Then close all those windows and start up Address Book, then test.