Important to know for all trouble shooting: What system are you running? Plenty of memory?
1. Is your software up to date? (Since I hear this is a common problem check for a trackpad-update, fix.
2. Did you do all the usual?: Restart your iMac. Use Disk Utilities / repair Disk (if you have: repair Permissions).
Start up in safe boot (safe mode) and back in normal again.
Turn Bluetooth off and on again. Make sure your trackpad and iMac is discoverable.
I remember reading: If scrolling is absent in Trackpad settings go to system preferences / Accessibility scroll down to mouse,trackpad.
Let me know and post back.
Walter