The Time Tracking Feature is not available for Help Desk cases as the system is designed in this way. More information about Time Tracking can be found in SuiteAnswers article Tracking Time on cases and Tracking Time on CRM Tasks.
As alternate solution, user has to use standard or custom Case Form instead. This can be done under List > Support > Cases > New > change the form in Custom Form field into any Case From.
Note: Enhancement 198901 will allow usage of time tracking for Help Desk Cases.