$ uname -a Linux raspberrypi-yau 3.18.11-v7+ #781 SMP PREEMPT Tue Apr 21 18:07:59 BST 2015 armv7l GNU/Linux
Know the version 3.18.11-v7+ and the number 781.
Download the corresponding driver, which is provided by Raspberry Pi official service.
$ cd /tmp $ wget https://dl.dropboxusercontent.com/u/80256631/8192eu-3.18.11-v7-781.tar.gz
Note the version 3.18.11-v7 and the number 781 in the name of the downloaded file.
Then unpack and install.
$ tar zxvf 8192eu-3.18.11-v7-781.tar.gz $ install.sh
It installs a configuration file and a .ko module file, and depmod it. After installation, the downloaded file and unpacked files will be removed.
Then write some setting words into /etc/network/interfaces, and reboot and find if it works for wireless network.