If you tried to sign up to the scirate implementation I linked to yesterday, you will likely have noticed that user activation didn't work. The cause was a stupid programmer mistake, combined with a test suite that didn't really test what it should have.
It should now work as intended. I've also manually marked all the accounts as active, so if you've already signed up, you should be able to sign in.
If you've signed up but not received a confirmation email, that is a bigger problem -- please let me know if this describes you.
Sorry!
Bill