To schedule work and/or book hours for another employee, you need to have rights on its calendar.
The employee needs at least to assign the role of 'Editor' for the one who books hours and needs at least to assign the role of 'Commentator' to the one who schedules hours.
The first time the role need to be even 'Owner', unless the one who books hours has synchronized the hours already one time in the past.
If you do not see categories, you have worked the first time without the rights as ‘Owner’. In that case you need to assign again 'Owner' rights to one who books the hours.
Moreover, a person whose calendar is not available in the public folders has to login into the application at least once so that the labor types become available as categories in Microsoft Outlook. If someone has his calendar available in the public folders, this will not be necessary: in that case the categories are used of the person who registers the hours in the calendar.
To set up rights properly on behalf of others, you can create a profile for that user. Go to the configuration screen and select ‘Email’. Then choose 'Profiles' and enter a profile for your colleague which you will use to assign in his name rights to others.