If you know the title of the activity or keywords that might help find it, you can search PBS KIDS. For example, you might do a general search for "cookie recipes" or a more specific search for "Cyberchase railroad" (if you were looking for the Cyberchase Railroad Repair activity). If you don't find what you want the first time, try a different combination of words.
If you know which site has the activity you're looking for, you can also check the homepage of that site to see if they have a program-specific site map or search area.