Do I just follow the directions for setting it up with MythTV or not?
I'm kinda confused
That's all I did. As far as I know there is no direct support for firewire in LinuxMCE, which is what I think Thom is suggesting, and is likely the right way to do it. I'm not sure what will happen if you re-run the setup wizard, or when you upgrade the system but I do know if you just set it up within MythTV you will be able to watch tv and record via firewire. I suspect you will not be able to use it with the "live TV" option in LMCE (different from watching live tv in Myth), but for a limited MythTV only tuner it works.
Before Thom scolds me for not doing it the LinuxMCE supported way let me say this probably isn't a LinuxMCE supported way to do it. However, there are a lot of non-programmers using LinuxMCE, including myself, and sometimes we need to step outside the rules a bit to get the functionality we want. Assume the usual disclaimers about this possibly messing up lmce, your house burning down, etc.