OP:
Did you already set up a "new account" on the iMac? That could be a mistake.
Be aware that you could start having problems if you try to "migrate" from the old MacBook.
I'll provide you with a way to correct this, if you care to follow my instructions.
I suggest you print this reply out, and "check steps off" as you follow along.
First, DO use CCC to clone the contents of the MacBook to an external drive.
But DON'T connect it to the new iMac, not yet.
Assuming you've already created a new account on the iMac, do this:
1. In your new account, go to the Users & Groups pref pane in System Preferences.
2. Click the lock icon and enter your password.
3. Now, click the "+" sign (above the lock icon). It will prompt you to create a new account (yes, you want to do this)
4. Give the new account administrative privileges.
5. Name it "temp502" and make the password "temp502" so you can remember it.
6. Create the new temp502 account.
7. Now, LOG OUT of your existing account, and LOG INTO your temp502 account.
8. Now, again click the lock icon and enter your password
9. Next, you want to click ONE TIME on the first new account you created (the one other than the temp502 account) to select it.
10. Now, click the "-" sign (above the lock icon) to DELETE the other (first) account. Again, YES, you want to do this (I'll explain why later)
11. You want to remove the account entirely (DON'T "archive" it).
12. When done, the only account on the new Mac will be the temp502 account.
Now, you're ready to connect the CCC cloned backup drive. Do this.
Next, open Migration Assistant (all other apps will quit).
It will ask for a password, use the "temp502" password.
Now, "aim" Migration Assistant at the new iMac.
Select what you want to "bring over" (I would select everything).
Then, let MA "do its thing".
This will take a while.
When done, you should be presented with a login screen and you can log into your account with your "old" password (the one you were using previously on the MacBook).
Everything should now look "as it did before", except you'll be on the new iMac.
KEEP READING.
WHY you needed to create the temp502 account above:
Accounts on the Mac have their own "ID numbers", that are normally hidden from the user.
The "first account created" will always have the ID "501", and be the primary account.
BUT... if you booted up the iMac and created a new account, and it became the 501 account, when you tried to migrate data from the MacBook, it would "come over" with a DIFFERENT account number (502, etc). Things wouldn't be right. You could start having "permissions problems".
SO...You needed to create a TEMPORARY "temp502" account first.
Now, you can log into the temp502 account and then DELETE the first, "501" account.
Now, when you migrate, the migrated account will "fall into" the 501 slot, and things will be as they should.
All your stuff will be there, no permissions problems.
Got it...?
After all this is done, you can delete the "temp502" account if you wish.