First, for reference, here's the original post from Facebook:
Simply text "otp" to 32665 on your mobile phone (U.S. only), and
you'll immediately receive a password that can be used only once and
expires in 20 minutes. In order to access this feature, you'll need a
mobile phone number in your account.
Notice that it says "U.S. only" and that you'll need to have registered a mobile phone number for your account already. The wording here is somewhat different from what you quoted in your post. The date for this is October 2010. I don't know if yours is more recent or not.
I found a step-by-step guide. It is not recent, but it doesn't seem as though the procedure has changed. It was written by someone that does information security for a living, and has very detailed instructions for what to do, in order to get a Facebook One Time Password.
Note that it is an OTP not an OPT as the questions mentioned in one part. That is important!
I think I found your source for getting a Facebook one time password, which may be more recent, as it includes a list of supported carriers outside the U.S. It gives
- directions for getting a code by email for an OTP if you do not have a confirmed mobile number, and
- directions for getting an 8-character OTP if you do have a confirmed mobile number.
I would suggest confirming your mobile number, as it seems more likely to work right.