Killing floor punk'd

Summer Sideshow 2011

The ones down the bottom of the list as they are the newest ones. (For Mrs. Foster)

And for Mr. Foster...well, I'm not so sure. I did unlock that skin 2 years ago so I cant tell which ones you have to do this year but I had to do the ones that basically say "kill circus creature A with weapon b" so look for achievement that involve circus themed zeds.

http://kf-wiki.com/wiki/Achievements

My friend was trying to unlock it too, and I think he mentioned that the Summer 2011 achievements were bugged, so you might want to look into that before spending too much time trying to get them.