If the Tattoo on Merrill doesn't appear. Go to your Creation kit (you should of gotten this for free when you got it on steam)it will be in tools.
Go to File. Data select Skyrim and Update. Then Merrill and put it as active file. After this wait for it to load (will take a while)
If it asks something just hit Yestoall. Now go to Breton, Female, Scroll or search for Merrill. Now select her. hit Ctrl and F4
This should make her Tattoo appear. 

There are videos to help or just ask me for more details.