Go into mail under reminders you will see notes. Expand notes so you can see "On My Mac" and "iCloud" and any other folders you may have. Choose the folder that has the notes that you want to move. Then pick the notes themselves, this can be done individually or you can select more than one. Once you have all the ones you want to move Right-click or choose Message from the menu bar at the top and select Move To -> Notes -> iCloud. Now all you notes will move move to the iCloud folder and within seconds they will change on all your devives sync'd in the cloud.
If yoiu're running Mail 6 on ML, then you may need to go in to an older version of mail where notes sit within there. Open it up ( I guess you may need to do a back or something.. but if you have a new computer and still have the old, the process is easy)
Basically, make sure you have set up icloud in mail, and your notes hsould be in there sitting in notes on your computer mail folder. You should see Icloud just below. Simply drag the notes from you computer to notes on Icloud..
Wait for it to sync and happy days
Hope hti smakes sense.. maybe someone with a bit more tech knowledge may explain it better