I know he still works in the same building, and he's right across the hall from where SNL is broadcasted from. He's in studio 8G, while SNL's in 8H. I THINK (I'm not sure, I could be totally wrong) that the address would go something like:

Seth Meyers
? Late Night with Seth Meyers
Studio 8G
30 Rockefeller Plaza
New York, NY 10112

I'm not sure if you'd even need to put the Studio number in there or not, I'm sure either way it'd work. I'm not completely sure about the address, but I'd think, given SNL's info on here (I didn't check Seth's page), that it would be that, formatted in that way or another. I could be wrong. Best bet is to do some research on other shows that film in that building because i know most of those shows are filmed at 30 Rock. I'd ask around though. Hope I helped!

I don't know but I would really like to know as well because I missed my chance to send to him while he was on SNL.