cse4k12.org is supposed to be the main URL for the website and cse4k12.com is supposed to simply redirect to .org.
Unfortunately, it looks like things are set up screwy and .org no longer redirects to the right place. I'll have to fix that but in the meantime all of the cse4k12.org links in the blog were not working.
Temp fix: change all the links to .com. I'll change them back once I fix the issue with .org, but for now if you have problems with any .org address, just change the "org" to "com" and it should work.
[Update: This has been fixed. All .org links are working again.]