Using a 2nd Ed adventure and I can't figure it out. Found somewhere online that said Threat is how many dice you add to each test, others sources refer to it as a pool but don't elaborate, but that has worked out both overpowered and underpowered respectively. I've given up trying to search for an answer, could someone please just spell out how it works for me?

In SR2, Threat Rating meant that you add a number of dice equal to the rating to EACH test, regardless if offensive or defensive. They do not need to refresh, those dice are always available. It also means the NPC doesn't get pools anymore.
It does not add to initiative, though.
Professional Rating is the point at which an NPC will break of combat and flee or beg for mercy.
1 means L wound, 2 M, 3 S and 4 that he will die before giving up.

Thank you.

Looks like I'm going to have to adjust the ratings, I'm not sure at what karma level a team can actually be expected to survive Harlequin's Back run vanilla, but it's higher than this team.

We took it on at around 75 to 100 karma. Been awhile (1999 or 2000) since I ran that so it is hard to remember. I do remember that you need a fairly experienced set of runners for it though. Also, we had a lot fun playing it and it is a great example of an astral quest. Hopefully that isn't a spoiler after a decade or more.