For those that might have difficulty registering for Thai the Sims 3. For the access code it is necessary for the characters to be in Thai.
It might take you a few tries but the Virtual Thai Keyboard is very helpful in that regard. You can find it here.
All you need to do is look at the letters for the access code and find the correct one on the virtual keyboard. Type them into the virtual keyboard one at a time and then copy and paste it in the code section. Rinse and repeat for each access code digit.
It may take you a few tries but eventually you will be able to access Thai the Sims 3 and all of its downloads.