If you want to gain root access on your Sony Ericsson Xperia Play, this is a pretty simple guide but you will need to have the bootloader unlocked, instructions are given below.
1. First, Unlock Bootloader of Xperia Play – Follow this guide.
2. Download this package: http://www.multiupload.com/U9Z2YJ2FAJ
3. Open Command Window (Start->Run->cmd) / or Terminal in Linux/Mac
Now copy the downloaded package to the location of fastboot you used for unlocking.
Enter the following command:
fastboot flash system YOURDOWNLOADEDFILE.img
fastboot -i 0x0fce flash system YOURDOWNLOADEDFILE.img
fastboot-mac -i 0x0fce flash system YOURDOWNLOADEDFILE.img
Now wait until it’s done.
3. REBOOT and you are good to go..