click 5 times SETTINGS>ABOUT PHONE>MODEL NUBMER and then in DEVELOPER OPTIONS, choose USB SETTINGS> CHARGING|MTP.

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