You can assign an activity to a task in the Task Details page, opened from the Define Work Plan view.
To associate an activity with a task:
Open the desired project.
Click Edit Work Plan.
The work plan Schedule View opens.
Select the desired task and click the Task Details icon.
The Task Details page opens.
In the Task Details page, select an Activity.
Note: If Time Management is being used, after a task with an activity is added to a time sheet and the time sheet is saved, the activity for that task cannot be changed.