I do not know the address, but look on the net for Clan Donald "seat" in Scotland.
Other wise she is in the Scottish Peers (Gentry)list.
Try the "Scots Ancestry Reseach Soc" at, email@example.com
or the web sight: http://www.royalmile,com/scotsancestry/
this was set up in 1945 to assist folk of Scottish decent to trace their Ancestry.