In many, if not most, Seinfeld episodes, a Macintosh computer can be seen in a small room at the back of Jerry's apartment. Did Jerry or any other character ever use this computer on-screen? If not, are there any references in the show to it having been used off-screen?
Yes, at least once. Jerry is using his computer in The Stall (Season 5, Episode 12) when Kramer walks in and asks to use the phone in his bedroom. Here's the GIF from the ep (I verified it myself):