After you purchase the Medal of Honor, you will need to do the following actions:
1) Go to Achievements (F4) > Special Achievement (the 5th red bookmark on the left) > "Receive Rewards" on the right page.
2) You will receive a Class Unlock Voucher. Use the item.
3) Once you have unlocked the Voucher, you should find the class in the "Advancement Info" window.
Also, some classes can only be advanced from the 2nd Advancement, which can be checked also in the "Advancement Info" window.
Please make sure that you are on your 2nd Advancement.