到http://www.mcu123.net/bbs/viewthread.php?tid=18973这个网站下载的MDK4.22a安装包安装好以后
将C51.rar解压后，放在根目录C;Keil下
然后在根目录中打开TOOLS.INI文件，在文件最后粘贴下以下一段话后保存即可。
[C51]
PATH="C:KeilC51"
BOOK0=HLPRelease_Notes.htm("Release Notes",GEN)
BOOK1=HLPC51TOOLS.chm("Complete User's Guide Selection",C)
TDRV0=BINMON51.DLL ("Keil Monitor-51 Driver")
TDRV1=BINISD51.DLL ("Keil ISD51 In-System Debugger")
TDRV2=BINMON390.DLL ("MON390: Dallas Contiguous Mode")
TDRV3=BINLPC2EMP.DLL ("LPC900 EPM Emulator/Programmer")
TDRV4=BINUL2UPSD.DLL ("ST-uPSD ULINK Driver")
TDRV5=BINUL2XC800.DLL ("Infineon XC800 ULINK Driver")
TDRV6=BINMONADI.DLL ("ADI Monitor Driver")
TDRV7=BINDAS2XC800.DLL ("Infineon DAS Client for XC800")
RTOS0=Dummy.DLL("Dummy")
RTOS1=RTXTINY.DLL ("RTX-51 Tiny")
RTOS2=RTX51.DLL ("RTX-51 Full")

至于破_解密码，自己到网站上下载注_册机吧，mdk4.22a跟mdk4.12的注_册可以通用的，破_解arm时选arm，破_解C51时选C51

点击此处下载 ourdev_691749UDKI7L.rar(文件大小:22.30M) (原文件名:C51.rar)