OK, I'll try, it's one of those things easier to show rather than explain though.
First, no idea why you are having trouble finding one. Call Phil at Fastlane, maybe he can help, or I can get the number for you if you can wait a day or two. I'm off work Thursday this week.
Near the bolt that holds the main filter in place is a black plastic hose, which comes to an end that looks like a black button (best way to describe it). The button is held down with a metal C shaped bracket, which in turn is held down by a bolt, 10mm head.
Remove the bolt and bracket, then you can both remove the plastic line (90 degree elbow, pull UP not back on the line), then the filter itself (the "button" the line goes into) can be pulled up out of the housing. It works just the opposite to reinstall it, no real trick involved. BUT take careful note of the O-ring I mentioned, I've never seen it come already installed on the new pre-filter or even supplied loose in the box with the pre-filter, it must be either bought new and installed, or at a minimum carefully removed from the old pre-filter and installed on the new one, I always want to replace it. IF you do neither and intall it without the O-ring, the injection pump will suck all the fuel out of the pre-filter and main filter, air will enter the system though the pre-filter and cause a no-start situation, plus all the air will need to be bled from the injection lines. A real mess. As you can tell, there was a rather steep learning curve associated with replacing filters on 606 motors. Save yourself the hassle and order a new o-ring for the pre-filter. I can get the number for that too if you need it.