Are you serious buddy?
?
Steps to reproduce:
1. Login
2. Click on Security Center
3. Click 'Add New' button below Yubikeys where it says 'No Yubikey has been linked'
Now tell me, that 'Add New' button - what URL does it go to?
Yup, it goes to
https://yubikey.mtgox.com/ which gives a 503 Error.
That's the same URL that you say doesn't exist - "I don't know where you got that URL but it's bogus so quit it"
I got it from trying to get a Yubikey in MtGox.
Still wanna say its bogus ...
Woah bigdude, ease down and just answer if you bought the yubikey from Mt. Gox.
-- If you did not, then as far as Mt. Gox is concerned -
you don't have one. I don't think they have a way to import user generated OAUTH HOTP keys from self purchased Yubikeys, and if they did I'd say it was a big security risk. There are special keys one can put into each Yubikey for a variety of authentication methods including OAUTH HOTP which I suspect Mt.Gox uses. These keys must be syncrhonized with the authenticating backend system.
I bought mine from Mtgox and it works perfectly. I have no idea what URL's they use when I sign in, maybe that's one of them - but who cares. It's easy and it works. Maybe they have a bug that makes this bogus page visible to non "Mtgox Yubikey" owners erroneously... If I recall, the "add new" is for use after you get your MtGox Yubikey in the post to essentially block a chosen transaction type unless the chosen authentication is successful.
And trust me, you don't need to worry about URL's or any technical mumbo jumbo if you get your key from Gox. For me, there was an easy link somewhere in the MtGox interface that took me to a shopping cart where I bought my key, selected payment, set up shipping, blah blah. FYI, while it's in transit, you'll notice that they update your account so you get a new "Press Yubikey" text field when you sign in. You can leave it blank until your Yubikey arrives, but after your first use, you'll only be able to sign in with your Yubikey (unless you bypass the feature in your security settings - something you'd need your Yubikey to do and which i wouldn't recommend).
The nice letter you get with your new MtGox Yubikey explains all of this.
-- If you HAVE ordered your key from Gox and it doesn't work - then you need to contact their support guys to fix it because unless you've tampered with your Yubikey, it should work barring errors on their end before they shipped it to you. Nobody on this forum would be able to help.