There are several ways to use the RadButton via the keyboard only:
Set its TabIndex property and use the Tab key to navigate to the desired button and then press the Enter key to invoke a click on the button.
Set its AccessKey property. This allows you to use the browser's shortcut combination to invoke the click ("Alt + AccessKey in IE and Chrome", "Shift + Alt + AccessKey" for Firefox, Shift + Esc + AccessKey for Opera).
Note: By default tab-access is disabled in Safari. To enable it, check "Preferences > Advanced > Press tab to highlight each item on a page".
To execute a selected Link button you may need to press the Enter key after the access key.