Nofollow in Directorypress

Hi, My website is based on Directorypress and I need all external links to be made 'nofollow'. I want do it myself but need directions. I have tried a couple of the plugins in Wordpress but either I'm doing something wrong or they don't work. I don't want to put a lot of money in this so all i need is someone helping me implement existing plugins from WordPress which will WORK. Only top rated freelancers will be taken into consideration. (my max is $35.00)