If you chose to enter time against an activity in the Timesheet Grid (Grid displaying activities down the left hand side) you will be able to define which project this activity was against. Use the drop down list to select the project which relates to this activity. However, if you chose to enter time against a project in the Timesheet Grid (Grid displaying projects down the left hand side), you will need to select which activity you need to log against this project from the drop down list.
Note:
The Projects (or Activities) contained in the drop down list are defined by your own project and activity restrictions.