puzzled by the chapter of "LVDS Interface with the Use External PLL Option Enabled"

It's a pain. What I've done in the past is make it with the PLL enabled, then copy the settings. I've lobbied to try and get the megawizard to always generate it with the PLL external and hooked up correctly. It would save a lot of pain.

And don't forget that when you create it with the PLL internal, run "derive_pll_clocks". You will see multicycles added that let it easily make timing. I think that's what this load signal does, is get it a few cycles early so there is plenty of setup and hold margin.

Again, I've never completely figured it out from a general sense, but have found copying what they do internally seems to work.