To change your MCC candidate code, login to your CaRMS Online account and go to "My Information -> physiciansapply.ca transfers”.
From this page, click the 'MCC acknowledgement' bar. In the MCC Candidate Code field, you will be able to enter the missing digit and then press the 'Acknowledge' button. Once this is done, this will save your 10 digit number (no dashes). Please note that copy/pasting the code with the hyphen will result in a digit being left out.