Notes from my iMac do not appear on my iPad or iPhone, what could be wrong?
This is most easily done through iCloud, which should be the same account you sign in to the App Store with.
Go to System Preferences > iCloud. If you haven't already set up your iCloud account on your Mac, simply click on iCloud, and enter the username and password.
In the right side field, scroll down to "Notes" and tick the checkbox next to it. You may need to wait 5 or so minutes for iCloud to update.
On your iPad and iPhone, go to Settings > iCloud, scroll down to Notes and turn on the slider.
For additional setup info: https://www.apple.com/icloud/setup/mac.html
The Notes Application on the Mac has local (on the Mac only) and iCloud notes.
On the Menu Bar click View > Show Folders List if it is hidden. If you have iCould set up, you should see 2 Lists: One for iCloud, the other called On My Mac.
You can drag individual Notes (not the folder) from the Notes On My Mac to the Notes list on iCloud. Then you should be able to see/edit them on your iOS devices.