I am not sure what you mean. The tab key works to move to the next field of the dialog for me. Please clarify and specify what hardware you are using and what operatin system.
The dialog was changed to support OS X better. There may be a way to configure OS X so that the tab key (or other key) selects buttons using easy access or the equivalent.
Why not just use the enter key for 'OK' and the esc key for 'Cancel'?