What speed DDR3 RAM you have that you need to locate XMP?
Do note that your MoBo only supports DDR3 RAM speeds of 1066 Mhz, 1333 Mhz and 1600 Mhz. So, if you have any faster RAM in the system, e.g 2400 Mhz, don't expect it to run at 2400 Mhz. Most what you can get out of RAM is 1600 Mhz.
MoBo specs: https://www.asus.com/Motherboards/H81MK/specifications/
As far as where you can change RAM speed, look into MoBo manual, page 2-10. Locate "DRAM frequency", since under there you can select RAM XMP if the RAM has XMP.
MoBo manual:
http://dlcdnet.asus.com/pub/ASUS/mb/LGA1150/H81M-K/E8601_H81M-K.pdf