You can enable two-step verification by signing into your dashboard, clicking the Settings Menu (your initials in the upper right hand corner) and select Personal Settings
Click on Two Step Verification Off next to the phone number field
Click Turn On Two Step Verification
Confirm the phone number and click confirm for a code to be sent to your phone
Note: make sure the phone number in your account information is the number you want to link with two-step verification (aka your cell number).