Needs adapter for mossberg 500 - FAB Defense Handguard Accessory Rail System For Mossberg 500/590

I'm having a (explitive) of a time finding info on an adapter kit I'll need to use to install this thing on my Mossberg 500. The rails which extend out of my stock forend to engage the bolt are not compatible with the Fabtec Defense forend. I looked through the Fabtec website, but no info could be found. I don't want to just start buying pieces hoping they fit. You would think this is a problem other people have had, and they would offer a product in the suggestioned products section for this issue. Also there was zero paperwork or info on this thing in the bag it came in. (Expletive) irritating