Unfortunately, Nokia Map Loader is a royal pain in the behind. It requires you to install loads of stuff, only works on windows, and adding new maps means that you have to download the maps you already had all over again.
Fortunately you don't need Nokia's maploader-from-hell:
Works on any operating system with any browser, and you don't need to install anything. Now you can finally add maps on locked-down computers like at work, in school, internet cafes, or straight from your phones built-in browser.