Weird but you said Garageband is working ok?? I know there's an issue with the installers after 2.3 and MacOS and the security settings. With High Sierra, you have to start allowing software to access the audio inputs. When you run an app for the first time it will ask you for permission to use the microphone and you have to tell it yes or it cant use the inputs at all. I started right at 2.5 so I didnt notice it until the day I actually wanted to sample something with it. I found the answer here, but I had to uninstall 2.5, install 2.3, run it once so I could allow it access, then upgrade back to 2.5 and now it works. For whatever reason (theyre on a roll with this ****) Akai forgot to do this with the installers for 2.4 and 2.5 and the support folks act like they have no clue why its doing this when people ask about it on their own forum.
I never got any message about it not working though, it just doesn't respond at all when it does that. If Garageband is working, then that means at one point you allowed Garageband to access the inputs with it. You may just need to reinstall MPC 2.3 and upgrade from there. Akai unfortunately just updated their website last week so all of us who's download links used to say MPC 2.3 are now updated so we can only download 2.5. The Akai support folks have emailed people copies of 2.3 to fix it but if you need it I can upload it somewhere. Im not sure if thats allowed here or not, but its not like the Mac version is cracked so it won't do anyone any good without an iLok license hehe.