I'm pretty sure from my own game that it doesn't work either and I think I know why.
I found this (along with many other sites): http://answers.ea.com/t5/The-Sims-3/how-do-you-hang-lights-in-seasons/td-p/206446 which says that you cannot place holiday lights if there is a flat roof - an apartment obviously has a flat roof and therefore you cannot hang them up.
