You must beat the side mission to get the achievement you can just do it right away when you unlock the mission, if you want you can decrypt the floppy disk. The Code is random for everyone and is not needed for this achievement.
If you want to get the code use the highlighted text on the newspaper to find the letters of a city and then the tape will tell you the numbers that you use as your code. The encoded message will have two question mark spots that you have to find and the combination of those two numbers will give you the numbers of another city on the tape and that city is the word portion of the code. To solve the encoded message find a pattern in the black numbers and a pattern in the red numbers to fill in the blanks. For me the patterns were increase 1, increase 2, increase 3, etc. on black and +8 for each number on red.