It didn't work for me either, but it's something wrong with the overview/search script, not with the actual formula pages. If you type "site:digitaltruth.com formula" into Google, you'll get a variety of links to pages for individual formulae that still work. For example, here's D-76:
https://www.digitaltruth.com/data/formula.php?FormulaID=128
Thus if you want a specific formula, you may be able to find the page with google, but digitaltruth should be notified that their overview page is broken.