The first time the name Dan Buck appears on the 990 form of Alexis de Tocqueville Institution (AdTI) is in 2000 as 'Secretary' with an address in Washington, DC. On the last available form (for 2001), his position changed to 'director' but the address is still the same. On both 990 forms he is said to work there only 0.5 hours per week for a $0.00 salary. In 2004 his name can be found on contact page of AdTI (www.adti.net/contacting.html) in combination with his email address at AdTI. No more information about Dan Buck is available.
On a 1994 list of 'Advisory Boards of the Alexis de Tocqueville Institution'  for the section 'American Immigration Institute' a 'Daniel Buck' can be found on the advisory board with the extra description:
Administrative Assistant, U.S. Congress
It's not sure if this is the same person or not.