iTunes + Migration Assistant Trouble

Since often transferring an account brings across so many items with a specific structure it isn't designed to overwrite an existing account, it just creates a new account that is as close as possible an exact duplicate of the one moved. If all you had wanted to bring over was the iTunes library you could have simply copied the entire iTunes folder from the PC to the Mac. If there's nothing in the one in your account on the Mac simply copy it over the (empty) one there and start iTunes. If there were already things to keep in your mac library, copy it somewhere else and hold down the option/alt key while starting iTunes.

If you want to move the one already on the computer this should be possible but OSX made things horribly complicated with file permissions. You can edit these but honestly if it were me I would just do it over the way I instructed above if at all possible. Perhaps somebody else can help with the file permissions. I guess you can put the iTunes folder from the new account into the users > Shared folder, then get it out of there (this still isn't easy from my memory of trying to do similar in the past). You'll have to log into and out of various accounts unless you're handy with Unix administrator commands.