* click 5 times  on **SETTINGS>ABOUT PHONE>MODEL NUBMER** 

* then **DEVELOPER OPTIONS** will be added in main settings (in bottom). Enter there and choose **USB SETTINGS> CHARGING|MTP.**


([Source][1]) 


  [1]: http://en.code-bude.net/2013/03/05/how-to-activate-usb-mass-storage-and-developer-options-on-cyanogenmod-10-1/