I've been trying to get my modified people search result web part to behave for some time. My problem was that my custom properties wouldn't show up no matter what i did. I followed Henry Ong's guide to the T without success.

After some searching I finally stumbled upon this post and, more importantly, the final comment of the post wich solved the problem for me.

When adding custom properties to your search result you must use lower case in the XSL-editor!

If you use "BeginnerSkills" the column will not be found and you have to use "beginnerskills" as your parameter name - even if you called the column "BeginnerSkills" when you included it in the Results Query Options section.