To make X-Lite dial a number from Apple's AddressBook, Control-Click, or use the right mouse button, on the number and select "Dial with X-Lite" from the Context Menu popping up.
The Plug-In strips characters from phone numbers, which couldn't be handled by X-Lite.
A leading "+" is replaced by "00" which is appropriate for most European countries.
If that does not work in the country you are living in, feel free to alter the Plug-In. It could be opened with the ScriptEditor.