I was able to answer the question of why the same procedure that worked for the person in the thread was not working for you. As such these other responses are rather speculative.
I have problems getting multi-channel audio with 10.8 to work, in 10.6.8 it worked easily for me. Mind you I've only spent a day playing around and installing ML. Trying with a new board in the next week or so (the GB H77N mini-itx) so I might have some better luck. (I too have the X-Fi USB old version so I'll report back later with how it worked for me in a week or two - I haven't used in OSX yet)
>>...surely the kexts for it are installed?
You'd think so... but I'm not sure if the hardware inside is the same as it originally was (same chip, etc). Just because a 'working' kext makes the name pop up doesn't mean it will do the advanced features. Working and fully functional are 2 very different things.
Out of curiosity, what are you using to test the discrete channels? If you have a DVD with the THX logo it - 90% of the time - has a series of tests that lets you play one channel of audio at a time (quite handy).
If your audio format is PCM (uncompressed aka lossless) you can ONLY EVER get L/R coming out of an optical (or RCA/coax) SPDIF. If you're using a compressed format like Dolby or DTS then the SPDIF will be able to spit out 6 discrete channels (L,R,C,LFE,Ls,Rs aka 5.1), but I'm not sure if there's any advanced settings that have to be changed around with inside OSX or if there are specific ones to ML.
I assume you're not using the optical and instead using the combo L/R RCA jacks and remaining 2 headphone jacks on the X-FI (for those of you reading that last sentence and wondering: it's the way the product is set-up) for the 5.1. The only other thing I can think of right now is to test it under 10.6 or 10.7: not a lot of help really.
>>I brought this because I saw all USB sound card's work then when i found the guide thought great!
Yeah, but when saying a sound card works most people just mean L/R. As for Creative Labs, I gave up on them years ago after wasting too much time with their products. There was a time when on-board sound was sub-standard, but that's a while ago - especially since digital connections came out.
The real selling point of an external sound card now is you don't have to worry about stray voltage (giving hisses or other interference) on the mother board affecting when you're recording something via the analog ports.