Make your check payable to Theta Delta Chi Educational Foundation, and mail it, along with a short letter stating the purpose of your gift (e.g. donation in the memory of Lindsay Russell), to the following address:
Theta Delta Chi Educational Foundation, Inc.
214 Lewis Wharf
Boston, MA 02110
1-800-999-1847
More information on contacting TDC can be found here (click on "Contacts").
More information on Theta Delta Chi can be found here.
Lindsay's page on TDC can be found here.