Click on the Manage Students Tab and go to AdHoc Attendance Planner
Choose your Student from the Select a Student dropdown, click choose and use the left and right arrows, at the top of the table that comes up, to find the date of the club you wish to allocate them to.
The table will show each day of the week and what clubs are on this day.
Simply click on the club session you wish to allocate them to and click Yes.
Alternatively on the day of the club in Workspace > Clubs > Registers > selecting the club you are wishing to add the student to and choose, at the bottom of the register you have the option to add an student to this club session. If you add the student here it will add them to the register for the club on just this day.