DISCLAIMER: I didn't take any credit for this post. This post is direct copy-pasta from Stm lab's user Rodi post
Edit: Support for 8188EU chipsets already added into the Raspbmc kernel after latest hotfix update ~ http://www.raspbmc.com/2013/10/raspbmc-hotfix/
Grab the kernel module mirrored here, put it in wireless modules folder, depmod -a, modprobe 8188eu. Full step-by-step guide is in my previous post.