How do i find the ip of a user who is not online but who was online today ? without the user having made posts
Through Admin CP?
Just go to ACP and on the index just scroll down and enter in a username in the "Quick user finder" and if you find a match, just scroll down and you'll see "Registration IP: ...."
but this ip is when registered right? I want the exact ip of a specific date and time
I don't think that information is saved in MyBB. Try looking at your server logs.