I'm taking some of the characters from Maria Mercurial (and other 2nd edition books) and making them NPC's in my game, but I can't find any way to do it. Are the stats the same from 2nd to 3rd edition?

There are some major differences between the editions, and, just as with SR3, there's no surefire way to convert any characters that will not end in tears and disappointment.
Your best bet will probably be to take a good look at what the NPC in question does, and then rebuild him, from scratch, in SR4, implementing the honed, tried and true RPG mechanic of winging the shit out of it.

I sure as hell did that when we actually played the campaign.

I think GMing has been a valuable experience because it took my ability to wing it to an entirely new level!

OP: Honestly, unless you want details notes for yourself, there's not much reason to even mess with a conversion. Just eyeball it and FBTSOYP¹.

If you wanted more detail...
The quickest method is to approximate their abilities.
Agi/Rea = Quickness
Int/Log = Intelligence
Swap outdated cyber for the 2070's equivalent. Example: VCR = Control Rig
Broad SR2/SR1 skills like Firearms get turned into Firearms Group
And so on.

I think there's also a SR3 > SR4 conversion guide floating around somewhere.

¹ Fly By The Seat Of Your Pants

-paws

If you want to do it the /really/ fun way, you use the SR2->SR3 conversion rules in the back of SR3 and then the SR3->SR4 conversion rules on the Shadowrun website.

Which, t'be fair, probably won't net you anything better than eyeballing it.

Generally speaking you can whittel the Attributes down to two thirds of the SR1-3 versions. You'll have to add in Intuition & Logic (they replace Intelligence), Quickness becomes Agility, move Reaction from the special Attributes to physical Attributes category. Add in Edge as you see fit and Magic too - the old SR1-3 editions eqated that Attribute to the character's Essence (rounded down to next nearest whole number).

As for Skills, well treat SR1-2 general skills as 4th ED Skill Groups. SR1-2 skill concentrations and SR3 general skills equate to basic SR4 skills. any SR1-3 specialisations just treat as SR4 specialisations (IE: get to add +2 dice where applicable). As for the skill ratings, modify these as listed in the SR3 to SR4 conversion guide - I think its still up on Catalyst's errata webpage. Oh yeah, and add on anything you think will round out the NPC for Knowledge skills - too many times this is missing, forgotton, or just plain wrong when it comes to published NPCs.

With spells, simply remove the Force ratings and in some cases convert them to a similar corresponding SR4 ED version.

Gear is easy - simply choose what you want them to have, modify Essence for cyber-/bio-/nano- etc.- ware, and apply a bit of GM limits - IE: don't give them stuff you don't want your players to get ahold of from body looting; ensure that a NPC actually has sensible gear to do what they should be able to do.